Focal infection theory

For the epidemiology concept, see Focus of infection.

Focal infection theory is the historical concept that many chronic diseases, including systemic and common ones, are caused by focal infections. In present medical consensus, a focal infection is a localized infection, often asymptomatic, that causes disease elsewhere in the host, but focal infections are fairly infrequent and limited to fairly uncommon diseases.^[1] (Distant injury is focal infection's key principle, whereas in ordinary infectious disease, the infection itself is systemic, as in measles, or the initially infected site is readily identifiable and invasion progresses contiguously, as in gangrene.)^[2][3] Focal infection theory, rather, so explained virtually all diseases, including arthritis, atherosclerosis, cancer, and mental illnesses.^[4][5][6][7]

An ancient concept that took modern form around 1900, focal infection theory was widely accepted in medicine by the 1920s.^[3][7][8][9] In the theory, the focus of infection might lead to secondary infections at sites particularly susceptible to such microbial species or toxin.^[3] Commonly alleged foci were diverse—appendix, urinary bladder, gall bladder, kidney, liver, prostate, and nasal sinuses—but most commonly were oral. Besides dental decay and infected tonsils, both dental restorations and especially endodontically treated teeth were blamed as foci.^[3][7] The putative oral sepsis was countered by tonsillectomies and tooth extractions, including of endodontically treated teeth and even of apparently healthy teeth, newly popular approaches—sometimes leaving individuals toothless—to treat or prevent diverse diseases.^[7]

Drawing severe criticism in the 1930s, focal infection theory—whose popularity zealously exceeded consensus evidence—was discredited in the 1940s by research attacks that drew overwhelming consensus of this sweeping theory's falsity. Thereupon, dental restorations and endodontic therapy became again favored.^[3][7] Untreated endodontic disease retained mainstream recognition as fostering systemic disease.^{[3][7][10][11]} But only alternative medicine and later biological dentistry continued highlighting sites of dental treatment—still endodontic therapy, but, more recently, also dental implant, and even tooth extraction, too—as foci of infection causing chronic and systemic diseases.^[12] In mainstream dentistry and medicine, the primary recognition of focal infection is endocarditis, if oral bacteria enter blood and infect the heart, perhaps its valves.^[2]

Entering the 21st century, scientific evidence supporting general relevance of focal infections remained slim, yet evolved understandings of disease mechanisms had established a third possible mechanism—altogether, metastasis of infection, metastatic toxic injury, and, as recently revealed, metastatic immunologic injury—that might occur simultaneously and even interact.^[2][13] Meanwhile, focal infection theory has gained renewed attention, as dental infections apparently are widespread and significant contributors to systemic diseases, although mainstream attention is on ordinary periodontal disease, not on hypotheses of stealth infections via dental treatment.^[14][15][16] Despite some doubts renewed in the 1990s by conventional dentistry's critics, dentistry scholars maintain that endodontic therapy can be performed without creating focal infections.^[3][7]

Rise and popularity (1890s–1930s)

Roots and dawn

Germ theory

Hippocrates, in ancient Greece, had reported cure of an arthritis case by tooth extraction.^[3] Yet focal infection, as such, appeared in modern medicine in 1877, when Karl Weigert reported "dissemination of 'tuberculosis poison' ".^[17] The prior year's breakthrough by Robert Koch, a fellow German, had launched medical bacteriology—a set of laboratory methods to isolate, culture, and multiply a single bacterium of one species^[18]—whereby Koch announced discovery of the "tubercle bacillus" in 1882, fully premising the modern principle of focal infection.^[6][19] In 1884, William Henry Welch, tasked to design the medical department at the newly forming Johns Hopkins University, imported the German model, "scientific medince", to America.^[20]

As progressively more diseases drew an infectious hypothesis that led to a pathogen discovery, conjectures grew that virtually all diseases are infectious.^[21] In 1890, German dentist Willoughby D Miller attributed a set of oral diseases to infections, and attributed a set of extraoral diseases—as of lung, stomach, brain abscesses, and other conditions—to the oral infections.^{[6][22][23][24]} In 1894, Miller became the first to identify bacteria in samples of tooth pulp.^[25][26] Miller advised root canal therapy.^[3][6] Yet ancient and folk concepts, entrenched as Galenic principles of humoral medicine, found new outlet in medical bacteriology, a pillar of the new "scientific medicine".^[27] Around 1900, British surgeons, still knife-happy, were urging "surgical bacteriology".^[27]

Autointoxication

In 1877, French chemist Louis Pasteur adopted Robert Koch's bacteriology protocols, but soon directed them to developing the first modern vaccines, and ultimately introduced rabies vaccine in 1885.^[28] Its success funded Pasteur's formation of the globe's first biomedical research institute, the Pasteur Institute.^[28] In 1886, Pasteur welcomed to Paris the emigration from Russia by international scientific celebrity Elie Metchnikoff—discoverer of phagocytes, mediating innate immunity—whom Pasteur granted an entire floor of the Pasteur Institute, once it opened in 1888.^[29] Later the institute's director and a 1908 Nobelist, Metchnikoff believed, as did his German immunology rival Paul Ehrlich—theorist on antibody, mediating acquired immunity^[30]—and as did Pasteur, too, that nutrition influences immunity.^[29] Metchnikoff brought to France its first yogurt cultures for probiotic microorganisms to suppress the colon's putrefactive microorganisms, which allegedly fostered the colon's toxic seepage causing degenerative disease, the putative phenomenon termed autointoxication.^[27][29][31] Metchnikoff reasoned that the colon functions as a "vesitigal cesspool" that stores waste but is unneeded.^[32]

Abdominal surgery's pioneer, Sir Arbuthnot Lane, based in London, drew from Metchnikoff and clinical observation to identify "chronic intestinal stasis"—in lay terms, intractable constipation—presumably, "flooding of the circulation with filthy material".^[27] Reporting surgical treatment in 1908, Lane eventually offered total colon removal, but later favored simply surgical release of colonic "kinks", and in 1925, abandoning surgery, began promoting prevention and intervention by diet and lifestyle, how Lane secured his contemporary reputation as a crank.^[27][31] Since 1875, in the American state Michigan, physician John Harvey Kellogg had targeted "bowel sepsis"—an allegedly prime cause of degeneration and disease—at his health resort, Battle Creek Sanitarium.^[27] Having, in fact, coined the term sanitarium, Kellogg yearly received several thousand patients, including US Presidents and celebrities, at his huge resort, advertised as the "University of Health".^[27] But in the 1910s, as North American medical schools emulated the German model—that is, "scientific medicine"^[33]—medical doctors who recognized "focal infection" were hinting a scientific basis versus the older, alleged "health faddists" like medical doctor Kellogg and like minister Sylvester Graham.^[27]

Medical popularity

Hunter on "oral sepsis"

In 1900, British surgeon William Hunter blamed many disease cases on oral sepsis.^[6][34][35] In 1910, lecturing in Montreal at McGill University, Hunter declared, "The worst cases of anemia, gastritis, colitis, obscure fevers, nervous disturbances of all kinds from mental depression to actual lesions of the cord, chronic rheumatic infections, kidney diseases are those which owe their origin to or are gravely complicated by the oral sepsis produced by these gold traps of sepsis."^[6] Thus, he apparently indicted dental restorations.^[22] Incriminating their execution, rather, his American critics lobbied for stricter requirements on dentistry licensing.^[6] Still, Hunter's lecture—as later recalled—"ignited the fires of focal infection".^[36] Ten years later, he proudly accepted that credit.^[8] And yet, read carefully, his lecture asserts a sole cause of oral sepsis: dentists who instruct patients to never remove partial dentures.^[36][37]

Billings & Rosenow

Focal infection theory's modern era really began with physician Frank Billings,^[21] based in Chicago, and his case reports of tonsillectomies and tooth extractions that apparently cured infections of distant organs.^[36] Replacing Hunter's term oral sepsis with focal infection,^[7] Billings in November 1911 lectured at the Chicago Medical Society, and published it in 1912 as an article for the American medical community.^[38] In 1916, Billings lectured in California at Stanford University Medical School, this time printed in book format.^[39] Billings thus popularized intervention by tonsillectomy and tooth extraction.^[6] A pupil of Billings, Edward Rosenow held that extraction alone was often insufficient, and urged teamwork by dentistry and medicine.^[22] Rosenow developed the principle elective localization, whereby microorganisms have affinities for particular organs, and also espoused extreme pleomorphism, whereby a bacterium can drastically change form and perhaps evade conventional detection methods.^[36][40][41]

Preeminent recognition

Since 1889, in the American state Minnesota, brothers William Mayo and Charles Mayo had built an international reputation for surgical skill at their Mayo Clinic, by 1906 performing some 5,000 surgeries a year, over 50% intra-abdominal, a tremendous number at the time, with unusually low mortality and morbidity.^[27][42] Though originally distancing themselves from routine medicine and skeptical of laboratory data, they later recruited Edward Rosenow from Chicago to help improve Mayo Clinic's diagnosis and care and to enter basic research via experimental bacteriology.^[27][42] Rosenow influenced Charles Mayo,^[27] who by 1914 published to support focal infection theory alongside Rosenow.^[43][44][45]

At Johns Hopkins University's medical school, launched in 1894 as America's first to teach "scientific medicine", the eminent Sir William Osler was succeeded as professor of medicine by Llewellys Barker,^[46] who became a prominent proponent of focal infection theory.^[27] Although many of the Hopkins medical faculty remained skeptics, Barker's colleague William Thayer^[47] cast support.^[27] As Hopkins' chief physician, Barker was a pivotal convert propelling the theory to the center of American routine medical practice.^[27] Russell Cecil,^[48] famed author of Cecil's Essentials of Medicine, too, lent support.^[36] In 1921, British surgeon William Hunter announced that oral sepsis was "coming of age".^[8]

Although physicians had already interpreted pus within a bodily compartment as a systemic threat, pus from infected tooth roots often drained into the mouth and thereby was viewed as systemically inconsequential.^[49] Amid focal infection theory, it was concluded that that was often the case—while immune response prevented dissemination from the focus—but that immunity could fail to contain the infection, that dissemination from the focus could ensue, and that systemic disease, often neurological, could result.^[49] By 1930, excision of focal infections was considered a "rational form of therapy" undoubtedly resolving many cases of chronic diseases.^[5] Its inconsistent effectiveness was attributed to unrecognized foci—perhaps inside internal organs—that the clinicians had missed.^[5]

Dental reception

In 1923, upon some 25 years of researches, dentist Weston Andrew Price of Cleveland, Ohio, published a landmark book,^[3][50] then a related article in the Journal of the American Medical Association in 1925.^[51] Price concluded that after root canal therapy, teeth routinely host bacteria producing potent toxins.^[3] Transplanting the teeth into healthy rabbits, Price and his researchers duplicated heart and arthritic diseases.^[3] Although Price noted often seeing patients "suffering more from the inconvenience and difficulties of mastication and nourishment than they did from the lesions from which their physician or dentist had sought to give them relief",^[52] his 1925 debate with John P Buckley was decided in favor of Price's position: "practically all infected pulpless teeth should be extracted".^[53] As chairman of the American Dental Association's research division, Price was a leading influence on the dentistry profession's opinion.^[54] Into the late 1930s, textbook authors relied on Price's 1923 treatise.^[55]

In 1911, the year that Frank Billings lectured on focal infection to the Chicago Medical Society, unsuspected periapical disease was first revealed by dental X-ray.^[36] Introduced by C. Edmund Kells,^[56] dental radiography to feed the "mania of extracting devitalized teeth".^[57] Even Price was cited as an authoritative source espousing conservative intervention at focal infections.^[58] Kells, too, advocated conservative dentistry.^[56] Many dentists were "100 percenters", extracting every tooth exhibiting either necrotic pulp or endodontic treatment, and extracted apparently healthy teeth, too, as suspected foci, leaving many persons toothless.^[3][7] A 1926 report published by several authors in Dental Cosmos—a dentistry journal where Willoughby Miller had published in the 1890s—advocated extraction of known healthy teeth to prevent focal infection.^[59] Endodontics nearly vanished from American dental education.^[3][7] Some dentists held that root canal therapy should be criminalized and penalized with six months of hard labor.^[7]

Psychiatric promulgation

Near the turn of the 20th century, psychiatry's predominant explanations of schizophrenia's causation, besides heredity, were focal infection and autointoxication.^[60] In 1907, psychiatrist Henry Andrews Cotton became director of the psychiatric asylum at Trenton State Hospital in the American state New Jersey.^[61] Influenced by focal infection theory's medical popularity,^[27] Cotton identified focal infections as the main causes of dementia praecox (now schizophrenia) and of manic depression (now bipolar disorder).^[61] Cotton routinely prescribed surgery not only to clean the nasal sinuses and to extract the tonsils and the teeth, but also to remove the appendix, gall bladder, spleen, stomach, colon, cervix, ovaries, and testicles, while Cotton claimed up to 85% cure rate.^[61]

Despite Cotton's death rate of some 30%, his fame rapidly spread through America and Europe, and the asylum drew influx of patients.^[61] The New York Times heralded "high hope".^[61] Cotton made a European lecture tour,^[61] and Princeton University Press and Oxford University Press simultaneously published his book in 1922.^[62] Despite skepticism in the profession, psychiatrists sustained pressure to match Cotton's treatments, as patients would ask why they were being denied curative treatment.^[61] Other patients were pressured or compelled into the treatment without their own consent.^[63] Cotton had his two sons' teeth extracted as preventive healthcare—although each later committed suicide.^[61] In the 1930s, however, focal infection fell from psychiatry as an explanation,^[60] Cotton having died in 1933.^[61]

Criticism and decline (1930s–1950s)

Early skepticism

Addressing the Eastern Medical Society in December 1918, New York City physician Robert Morris had explained that focal infection theory had drawn much interest but that understanding was incomplete, while the theory was earning disrepute through overzealousness of some advocates.^[64] Morris called for facts and explanation from scientists before physicians continued investing so steeply in it, already triggering vigorous disputes and embittering divisions among clinicians as well as uncertainty among patients.^[64]

In 1919, the American Dental Association's forerunner, the National Dental Association, held in New Orleans its annual meeting, where C Edmund Kells, the originator and pioneer of dental X-ray,^[56] delivered a lecture, published in 1920 in the association's journal,^[65] largely discussing focal infection theory, which Kells condemned as a "crime".^[57] Kells stressed that X-ray technology is to improve dentistry, not to enhance the "mania of extracting devitalized teeth".^[57] Kells urged dentists to reject physicians' prescriptions of tooth extractions.^[66]

Focal infection theory's elegance suggested simple application, but the surgical removals brought meager "cure" rate, occasional disease worsening, and inconsistent experimental results.^[6] Still, the lack of controlled clinical trials, among present criticism,^[6] was normal at the time—except in New York City.^[61] Around 1920, at Henry Cotton's claims of up to 85% success treating schizophrenia and manic depression, Cotton's major critic was George Kirby, director of the New York State Psychiatric Institute on Ward's Island.^[67] As colleagues of Kirby, two researchers—bacteriologist Nicolas Kopeloff and psychiatrist Clarence Cheney—ventured from Ward's Island to Trenton, New Jersey, to investigate Cotton's practice.^[61]

Research attacks

In two controlled clinical trials with alternate allocation of patients, Nicolas Kopeloff, Clarence Cheney, and George Kirby concluded Cotton's psychiatric surgeries ineffective: those who improved were already so prognosed, and others improved without surgery.^[61][68] Publishing two papers, the team presented the findings at the American Psychiatric Association's 1922 and 1923 annual meetings.^[61][69] At Johns Hopkins University, Phyllis Greenacre questioned most of Cotton's data, and later helped steer American psychiatry into psychoanalysis.^[61] Antipsychotic colectomy vanished except in Trenton until Cotton—who used publicity and word of mouth, kept the 30% death rate unpublicized, and passed a 1925 investigation by New Jersey Senate—died by heart attack in 1933.^[61]

By 1927, Weston Price's researches had been criticized for allegedly "faulty bacterial technique".^[70] In the 1930s and 1940s, researchers and editors dismissed the studies of Price and of Edward Rosenow as flawed by insufficient controls, by massive doses of bacteria, and by contamination of endontically treated teeth during extraction.^[3] In 1938, Russell Cecil and D Murray Angevine reported 200 cases of rheumatoid arthritis, but no consistent cures by tonsillectomies or tooth extractions.^[3][71] They commented, "Focal infection is a splendid example of a plausible medical theory which is in danger of being converted by its enthusiastic supporters into the status of an accepted fact."^[6] Newly a critic, Cecil alleged that foci were "anything readily accessible to surgery".^[36]

In 1939, E W Fish published landmark findings that would revive endodontics.^[3] Fish implanted bacteria into guinea pigs' jaws, and reported that four zones of reaction consequently developed.^[3][72] Fish reported that the first zone was the zone of infection, whereas the other three zones—surrounding the zone of infection—revealed immune cells or other host cells but no bacteria.^[3] Fish theorized that by removing the infectious nidus, dentists would permit recovery from the infection^[3] This reasoning and conclusion by Fish became the basis for successful root-canal treatment.^[3] Still, endodontic therapy of the era indeed posed substantial risk of failure, and fear of focal infection crucially motivated endontologists to develop new and improved technology and techniques.^[7]

End of the focal era

The review and "critical appraisal" by Hobart A Reimann and W Paul Havens, published in January 1940,^[37] was perhaps the most influential criticism of focal infection theory.^[6] Recasting British surgeon William Hunter's landmark pronouncements of 30 years earlier as widely misinterpreted, they summarized that "the removal of infectious dental focal infections in the hope of influencing remote or general symptoms of disease must still be regarded as an experimental procedure not devoid of hazard".^[59] By 1940, Louis I Grossman's textbook Root Canal Therapy flatly rejected the methods and conclusions made earlier by Weston Price and especially by Edward Rosenow.^[73] Amid improvements in endodontics and medicine, including release of sulfa drugs and antibiotics, a backlash to the "orgy" of tooth extractions and tonsillectomies ensued.^[6]

K A Easlick's 1951 review in the Journal of the American Dental Association notes, "Many authorities who formerly felt that focal infection was an important etiologic factor in systemic disease have become skeptical and now recommend less radical procedures in the treatment of such disorders".^[74] A 1952 editorial in Journal of the American Medical Association tolled the era's end by stating that "many patients with diseases presumably caused by foci of infection have not been relieved of their symptoms by removal of the foci", that "many patients with these same systemic diseases have no evident focus of infection", and that "foci of infection are as common in apparently healthy persons as in those with disease".^[75][76] Although some support extended into the late 1950s,^[77][78] focal infection vanished as the primary explanation of chronic, systemic diseases,^[15] and the theory was generally abandoned in the 1950s.^[79]

Revival and evolution (1990s–2010s)

Despite the general theory's demise, focal infection remained a formal, if rare, diagnosis, as in idiopathic scrotal gangrene^[80] and angioneurotic edema.^[81] Meanwhile, by way of continuing case reports claiming cures of chronic diseases like arthritis after extraction of infected or root-filled teeth, and despite lack of scientific evidence, "dental focal infection theory never died".^[7] In fact, severe endodontic disease resembles classic focal infection theory.^[7][76] In 1986, it was noted that, "in spite of a decline in recognition of the focal-infection theory, the association of decayed teeth with systemic disease is taken very seriously".^[10] Eventually, the theory of focal infection drew reconsideration.^[79] Conversely, attribution of endocarditis to dentistry has entered doubt via case-control study, as the species usually involved is present throughout the human body.^[82]

Stealth pathogens

With the 1950s introduction of antibiotics, attempts to explain unexplained diseases via bacterial etiology seemed all the more unlikely.^[83] By the 1970s, however, it was established that antibiotics could trigger bacteria switch to their L phase.^[84] Eluding detection by traditional methods of medical microbiology, bacterial L forms and the similar mycoplasma—and, later, viruses—became the entities expected in the theory of focal infection.^[83][84] Yet until the 1980s, such researchers were scarce, largely due to scarce funding for such investigations.^[83]

Despite the limited funding, research established that L forms can adhere to red blood cells and thereby disseminate from foci within internal organs such as the spleen,^[85] or from oral tissues and the intestines, especially during dysbiosis.^[86][87] Perhaps some of Weston Price's identified "toxins" in endodontically treated teeth were L forms,^[88] thought nonexistent by bacteriologists of his time and widely overlooked into the 21st century.^[89] Apparently, dental infections, including by uncultured or cryptic microorganisms, contribute to systemic diseases.^{[90][91][92][93][88][87]}

Periodontal medicine

At the 1990s' emergence of epidemiological associations between dental infections and systemic diseases, American dentistry scholars have been cautious,^[79] some seeking successful intervention to confirm causality.^[3][94] Some American sources emphasized epidemiology's inability to determine causality, categorized the phenomena as progressive invasion of local tissues, and distinguished that from focal infection theory—which they assert was evaluated and disproved by the 1940s.^[3] Others have found focal infection theory's scientific evidence still slim, but have conceded that evolving science might establish it.^[2] Yet select American authors affirm the return of a modest theory of focal infection.^[95][96]

European sources find it more certain that dental infections drive systemic diseases, at least by driving systemic inflammation, and probably, among other immunologic mechanisms, by molecular mimicry resulting in antigenic crossreaction with host biomolecules,^[16][97][98] while some seemingly find progressive invasion of local tissues compatible with focal infection theory.^[98] Acknowledging that beyond epidemiological associations, successful intervention is needed to establish causality, they emphasize that biological explanation is needed atop both, and the biological aspect is thoroughly established already, such that general healthcare, as for cardiovascular disease, must address prevalent periodontal disease,^[97][99] a stance matched in Indian literature.^[100] Thus, there has emerged the concept periodontal medicine.^[16][79]

Dental controversies

During the 1980s, dentist Hal Huggins, sparking severe controversy, spawned biological dentistry, which claims that conventional tooth extraction routinely leaves within the tooth socket the periodontal ligament that often becomes gangrenous, then, forming a jawbone cavitation seeping infectious and toxic material.^[12] Sometimes forming elsewhere in bones after injury or ischemia,^[17] jawbone cavitations are recognized as foci also in osteopathy^[17] and in alternative medicine,^[101] but conventional dentists generally conclude them nonexistent.^[17] Although the International Academy of Oral Medicine & Toxicology claims that the scientific evidence establishing existence of jawbone cavitations is overwhelming and even published in textbooks, the diagnosis and related treatment remain controversial,^[102] and allegations of quackery persist.^[103]

Huggins and many biological dentists also espouse Weston Price's findings on endodontically treated teeth routinely being foci of infection,^[12] although these dentists have been accused of quackery.^[104] Conventional belief is that microorganisms within inaccessible regions of a tooth's roots are rendered harmless once entrapped by the filling material, although little evidence supports this.^[105] A H Rogers in 1976^[106] and E H Ehrmann in 1977^[107] had dismissed any relation between endodontics and focal infection.^[59] At dentist George Meinig's 1994 book, Root Canal Cover-Up, discussing researches of Rosenow and of Price, some dentistry scholars reasserted that the claims were evaluated and disproved by the 1940s.^[108][109] Yet Meinig was but one of at least three authors who in the early 1990s independently renewed the concern.^[59]

Boyd Haley and Curt Pendergrass reporting finding especially high levels of bacterial toxins in root-filled teeth.^[110][88] Although such possibility appears especially likely amid compromised immunity—as in individuals cirrhotic, asplenic, elderly, rheumatoid arthritic, or using steroid drugs—there remained a lack of carefully controlled studies definitely establishing adverse systemic effects.^[59] Conversely, some if few studies have investigated effects of systemic disease on root-canal therapy's outcomes, which tend to worsen with poor glycemic control, perhaps via impaired immune response, a factor largely ignored until recently, but now recognized as important.^[59] Still, even by 2010, "the potential association between systemic health and root canal therapy has been strongly disputed by dental governing bodies and there remains little evidence to substantiate the claims".^[59]

The traditional root-filling material is gutta-percha, whereas a new material, Biocalex, drew initial optimism even in alternative dentistry, but Biocalex-filled teeth were later reported by Boyd Haley to likewise seep toxic byproducts of anaerobic bacterial metabolism.^[111][112] Seeking to sterilize the tooth interior, some dentists, both alternative and conventional, have applied laser technology.^[112][113] Although endodontic therapy can fail and eventually often does,^[105][114] dentistry scholars maintain that it can be performed without creating focal infections.^[3] And even by 2010, molecular methods had rendered no consensus reports of bacteremia traced to asymptomatic endodontic infection.^[7] In any event, the predominant view is that shunning endodonthic therapy or routinely extracting endodontically treated teeth to treat or prevent systemic diseases remains unscientific and misguided.^{[3][109][115]}

Footnotes

1. See, for example, David Schlossberg, ed, Clinical Infectious Disease, 2nd edn (Cambridge University Press, 2015), and Yomamoto T, "Triggering role of focal infection...", in Harabuchi Y et al, eds, Recent Advances in Tonsils and Mucosal Barriers of the Upper Airways (Karger, 2011).
2. Jed J Jacobson & Sol Silverman Jr, ch 17 "Bacterial infections", in Sol Silverman, Lewis R Eversole & Edmond L Truelove, eds, Essentials of Oral Medicine (Hamilton Ontario: BC Decker, 2002), pp 159–62.
3. J Craig Baumgartner, José F Siqueira Jr, Christine M Sedgley & Anil Kishen, ch 7 "Microbiology of endodontic disease", in John I Ingle, Leif K Bakland & J Craig Baumgartner, eds, Ingle's Endodontics, 6th edn (Hamilton Ontario: BC Decker, 2008), p 221–24.
4. Paul R Stillman & John O McCall, A Textbook of Clinical Periodontia, (New York: Macmillan Co, 1922), "ch 18 Focal infection".
5. Graham D (1931). "Focal infection". Canadian Medical Association Journal. 25 (4): 422–4. PMC 382689. PMID 20318466.
6. Pallasch TJ, Wahl MJ (2000). "The focal infection theory: Appraisal and reappraisal". Journal of the California Dental Association. 28 (3): 194–200. doi:10.1080/19424396.2000.12223068. PMID 11326533. S2CID 42277199.
7. Nils Skaug & Vidar Bakken, ch 8 "4Systemic complications of endodontic infections", subchapter "Chronic periapical infections as the origin of metastatic infections", in Gunnar Bergenholtz, Preben Hørsted-Bindslev & Claes Reit, eds, Textbook of Endodontology, 2nd ed. (West Sussex: Wiley-Blackwell, 2010), pp 135–37.
8. Hunter W (1921). "The coming of age of oral sepsis". British Medical Journal. 1 (3154): 859. doi:10.1136/bmj.1.3154.859. PMC 2415200. PMID 20770334.
9. Wisner FP (1925). "Focal infection, a medico-dental problem". California and Western Medicine. 23 (8): 977–80. PMC 1654829. PMID 18739726.
10. James M Dunning, Principles of Dental Public Health, 4th edn (Cambridge MA: Harvard University Press, 1986), ch 13 "Dental needs and resources", § "Systemic infection of dental origin", p 272–73.
11. Gavett G, "Tragic results when dental care is out of reach"4, PBS Frontline website, 26 Jun 2012
12. Hal A Huggins & Thomas E Levy, Uninformed Consent: The Hidden Dangers in Dental Care (Charlottesvi4lle VA: Hampton Roads Publishing, 1999), ch 12 "The cavitation" & ch 13 "Focal infection".
13. Nchaitanya Babu & Andreajoan Gomes (2011). "Systemic manifestations of oral diseases". Journal of Oral and Maxillofacial Pathology. 15 (2): 144–7. doi:10.4103/0973-029X.84477. PMC 3329699. PMID 22529571.
14. Goymerac B, Woollard G (2004). "Focal infection: A new perspective on an old theory". General Dentistry. 52 (4): 357–61. PMID 15366304.
15. Shantipriya Reddy, Essentials of Clinical Periodontology and Periodontics, 2nd edn (New Delhi: Jaypee Brothers Medical Publishers, 2008), ch 13 "Periodontal medicine", esp pp 115–16.
16. Pizzo G, Guiglia R, Lo Russo L, Campisi G (2010). "Dentistry and internal medicine: From the focal infection theory to the periodontal medicine concept". European Journal of Internal Medicine. 21 (6): 496–502. doi:10.1016/j.ejim.2010.07.011. PMID 21111933.
17. Leon Chaitow, Cranial Manipulation: Theory and Practice, 2nd edn (Edinburgh, London, New York, etc.: Elsevier, 2005), pp 348–49 & 350–51.
18. In 1876, employing innovative bacteriology protocols more stringently reductionist than previous bacteriology techniques, Koch confirmed that a recurrently suspected bacterial species—later named Bacillus anthracis—causes anthrax, which thus became the first mammalian disease scientifically explained as infectious.
19. Gibbons RV (1998). "Germs, Dr. Billings, and the theory of focal infection". Clinical Infectious Diseases. 27 (3): 627–33. doi:10.1086/514705. PMID 9770165.
20. Barry D. Silverman, "William Henry Welch (1850–1934): The road to Johns Hopkins", Proc (Bayl Univ Med Cent), 2011 Jul;24(3):236–242.
21. R V Gibbons, "Germs, Dr. Billings, and the theory of focal infection", Clin Infect Dis, 1998 Sep;27(3):627–633.
22. Barnett ML (2006). "The oral-systemic disease connection: An update for the practicing dentist". Journal of the American Dental Association. 137 Suppl: 5S–6S. doi:10.14219/jada.archive.2006.0401. PMID 17012729.
23. Miller WD (1891). "The human mouth as a focus of infection". The Lancet. 138 (3546): 340–342. doi:10.1016/S0140-6736(02)01387-9.
24. Willoughby D Miller, The Micro-Organisms of the Human Mouth: The Local and General Diseases Which Are Caused by Them (Leipzig: Verlag von Georg Thieme, 1892).
25. Shanon Patel & Justin J Barnes, "Introduction: How has endodontics developed?", in Shanon Patel & Justin J Barnes, eds, The Principles of Endodontics, 2nd edn (Oxford: Oxford University Press, 2013), pp 4–5.
26. Miller WD (1894). "An introduction to the study of the bacteriopathology of the dental pulp". Dental Cosmos. 36: 505–28.
27. Andrew Scull, Madhouse: A Tragic Tale of Megalomania and Modern Medicine (New Haven: Yale University Press, 2005), pp 33–37.
28. Caroline Barranco, "The first live attenuated vaccines", Nature Portfolio, Springer Nature Limited, 28 Sep 2020.
29. Alfred I Tauber & Leon Chernyak, Metchnikoff and the Origins of Immunology: From Metaphor to Theory (New York: Oxford University Press, 1991), pp viii, 11.
30. Immunology's early feuding over whether immunity is innate or is acquired reflected limited perspectives. As later elucidated, antibody molecules, which are secreted by activated B cells, mediate but one arm of acquired immunity, whose other arm is mediated by killer T cells. Acquired immunity's two principal mediators, B cells and T cells, are subsets of lymphocytes—themselves a subset of white blood cells, also known as leukocytes—as their base of residence is peripheral lymphoid tissues. On the other hand, innate immunity is mediated not only by phagocytes but also by a third subset of lymphocytes: natural killer cells. Further, innate immunity also includes soluble components—complement proteins and, more recently identified, innate antibody secreted by B cells—while yet other lymphocytes, specifically helper T cells, bridge innate immunity and adaptive immunity.
31. Chen TS, Chen PS (1989). "Intestinal autointoxication: A medical leitmotif". Journal of Clinical Gastroenterology. 11 (4): 434–41. doi:10.1097/00004836-198908000-00017. PMID 2668399.
32. Philip A. Mackowiak, "Recycling Metchnikoff: Probiotics, the intestinal microbiome and the quest for long life", Front Public Health, 2013 Nov 13;1:52.
33. Andrew H Beck, "The Flexner report and the standardization of American medical education", JAMA, 2004 May 5;291(17):2139–2140. Lester S King, "Medicine in the USA: Historical vignettes. XX. The Flexner report of 1910", JAMA, 1984 Feb 24;251(8):1079–1086. A I Tauber, "The two faces of medical education: Flexner and Osler revisited", J R Soc Med, 1992 Oct;85(10):598–602, especially pp 599–600.
34. Hunter, W (28 July 1900). "Oral sepsis as a cause of disease". British Medical Journal. 2 (2065): 215–6. doi:10.1136/bmj.2.2065.215. PMC 2462945. PMID 20759127.
35. William Hunter, Oral Sepsis as a Cause of Septic Gastritis, Toxic Neuritis, and Other Septic Conditions (London: Cassell & Co, 1901).
36. John I Ingle, PDQ Endodontics, 2nd edn (Shelton CT: People's Medical Publishing House, 2009), p xiv.
37. Hobart A Reimann & Paul Havens (1940). "Focal infection and systemic disease: A critical appraisal". Journal of the American Medical Association. 114. doi:10.1001/jama.1940.02810010003001.
38. Frank Billings (1912). "Chronic focal infections and their etiologic relations to arthritis and nephritis". Archives of Internal Medicine. IX (4): 484–498. doi:10.1001/archinte.1912.00060160087007.
39. Frank Billings, Focal Infection: The Lane Lectures (New York & London: D Appleton & Co, 1918).
40. Edward C Rosenow; Hazel Gray (1918). "Agglutination of the pleomorphic Streptococcus isolated from epidemic poliomyelitis by immune serum". Journal of Infectious Diseases. 22 (4): 345–378. doi:10.1093/infdis/22.4.345. JSTOR 30080461.
41. Edward C Rosenow, ch 43 "Elective localization of bacteria in the animal body", in Edwin O Jordan & I S Falk, eds The Newer Knowledge of Bacteriology and Immunology (Chicago IL: University of Chicago Press, 1928), pp 576–89.
42. "Mayo Clinic history", Mayo Clinic, Website access: 21 Sep 2013.
43. Billings F (1914). "Mouth infection as a source of systemic disease". Journal of the American Medical Association. 63 (23): 2024. doi:10.1001/jama.1914.02570230034010.
44. Mayo CH (1914). "Mouth infection as a source of systemic disease". Journal of the American Medical Association. 63 (23): 2025. doi:10.1001/jama.1914.02570230035011.
45. Rosenow EC (1914). "Mouth infection as a source of systemic disease". Journal of the American Medical Association. 63 (23): 2026. doi:10.1001/jama.1914.02570230036012. S2CID 72457427.
46. "The Lewellys Franklin Barker Collection", Alan Mason Chesney Medical Archives, Johns Hopkins Medical Institutions, Website access: 23 Sep 2013.
47. "The William S Thayer Collection Archived 2013-09-22 at the Wayback Machine", Repository Guide to the Personal Papers Collections of Alan Mason Chesney Medical Archives, Johns Hopkins Medical Institutions, Website access: 21 Sep 2013.
48. Copeman WSC (Sep 1965). "Russell L Cecil". Annals of the Rheumatic Diseases. 24 (5): 502. doi:10.1136/ard.24.5.502. PMC 1031019.
49. Craig CB (1914). "Peridental infection as a causative factor in nervous diseases". Journal of the American Medical Association. 63 (23): 2027. doi:10.1001/jama.1914.02570230037013.
50. Weston A Price, Dental Infections, Oral and Systemic, Vol 1 & Vol 2 (Cleveland: Penton Publishing, 1923).
51. Price WA (1925). "Dental infections and related degenerative diseases". Journal of the American Medical Association. 84 (4): 254. doi:10.1001/jama.1925.02660300012006.
52. Weston A Price, Dental Infections, Oral and Systemic, Vol 1 (Cleveland: Penton Publishing, 1923), p 488.
53. Baumgartner et al, Ingle's Endodontics, 6th edn (Hamilton: BC Decker, 2006), p 286, not giving that precise summary, cite Buckley–Price debate on the subject: Price Weston A, Buckley John P (1925). "Resolved: that practically all infected pulpless teeth should be extracted". Journal of the American Dental Association. 12: 1468–524. doi:10.14219/jada.archive.1925.0307.
54. British Journal of Dental Science, 1928;72:101.
55. Examples: William H O McGehee, A Text-book of Operative Dentistry, 2nd edn (Philadelphia: Blackiston's Son & Co, 1936), pp 39 & 110; Louis V Hayes, Clinical Diagnosis of Diseases of the Mouth: A Guide for Students and Practitioners of Dentistry and Medicine (Brooklyn NY: Dental Items of Interest Publishing, 1935), p 389.
56. Kracher CM (2000). "C. Edmund Kells (1856-1928)". Journal of the History of Dentistry. 48 (2): 65–9. PMID 11794365.
57. ADA: "C Edmund Kells was a dental pioneer who championed the use of X-rays in dentistry during the late 19th century and early 20th century. 'The X-ray in dental practice' is a paper read by Dr Kells at a 1919 Association meeting in New Orleans. Much of the paper discusses focal infection theory, which Dr Kells argued was leading to the unnecessary extraction of teeth. He also made it clear that dental X-rays should be used to enhance dentistry, and not to encourage the 'mania for extracting devitalized teeth' " ["JADA Centennial: From the February 2013 issue of JADA", American Dental Association, Website access: 21 Sep 2013].
58. Journal of the Canadian Dental Association, 1935;1:451.
59. C Murray, ch 48 "Endontology and general systemic health", §§ "The 'focal infection' era" & "Effects of general systemic health on endodontics", in Michael Baumann & Rudolf Beer, eds, Endodontology, 2nd edn (New York: Thieme, 2010).
60. Noll Richard (Apr 2004). "Historical review: Autointoxication and focal infection theories of dementia praecox". World Journal of Biological Psychiatry. 5 (2): 66–72. doi:10.1080/15622970410029914. PMID 15179665. S2CID 41416327.
61. Wessely S (October 2009). "Surgery for the treatment of psychiatric illness: The need to test untested theories". Journal of the Royal Society of Medicine. 102 (10): 445–51. doi:10.1258/jrsm.2009.09k038. PMC 2755332. PMID 19797603.
62. Henry A Cotton, The Defective, Delinquent, and Insane: The Relation of Focal Infections to their Causation, Treatment and Prevention (Princeton/London: Princeton University Press/Oxford University Press, 1922).
63. Phil Fennell, Treatment Without Consent: Law, Psychiatry and the Treatment of Mentally Disordered People since 1845 (London & New York: Routledge, 1996), p. 120.
64. Robert T Morris: "The matter of focal infections is one of the very new subjects of the day which men are taking up with a great deal of interest but are going ahead perhaps with incomplete knowledge and not comprehending the range and scope of the entire subject; consequently this subject is falling into disrepute in certain fields because of the over-enthusiasm of some of the advocates of focal infection theory in relation to distant demonstration—endocarditis, rheumatism, gastric ulcer, cholecystitis, various forms of neuritis, etc. The philosopher, taking all evidence judicially, will eventually give the medical profession the basic facts and what is valuable in the subject. Right now one might utter a warning to the general medical profession against taking too active an interest in the subject." ["Address on medicine and surgery", American Medicine, 1919 Jan;25(1):17–23, pp 18–19].
65. C Edmund Kells, "X-ray in dental practice", Journal of the National Dental Association, 1920 Mar;7(3):241–72 [JADA provides the article free in two parts (1 Archived 2013-09-27 at the Wayback Machine & 2 Archived 2013-09-27 at the Wayback Machine)].
66. Jacobsohn PH, Kantor ML, Pihlstrom BL (2013). "The X-ray in dentistry, and the legacy of C. Edmund Kells: A commentary on Kells CE. The X-ray in dental practice. J Natl Dent Assoc 1920;7(3):241-272". Journal of the American Dental Association. 144 (2): 138–42. doi:10.14219/jada.archive.2013.0092. PMID 23372129.
67. Richard Noll, The Encyclopedia of Schizophrenia and Other Psychotic Disorders (New York: Facts on File, 2007), pp 170–170.
68. Shorter E (2011). "A brief history of placebos and clinical trials in psychiatry". Canadian Journal of Psychiatry. 56 (4): 193–7. doi:10.1177/070674371105600402. PMC 3714297. PMID 21507275.
69. Kopeloff Nicolas, Cheney Clarence O (Oct 1922). "Studies in focal infection: Its presence and elimination in the functional psychoses". American Journal of Psychiatry. 79 (2): 139–56. doi:10.1176/ajp.79.2.139.
    Kopeloff Nicolas, Kirby George H (Oct 1923). "Focal infection and mental disease". American Journal of Psychiatry. 80 (2): 149–91. doi:10.1176/ajp.80.2.149.
70. Henry W Crowe & Herbert G Franking, "Aetiology continued: Dental infections and degenerative diseases—a review and commentary", pp 23–32, Bacteriology and Surgery of Chronic Arthritis and Rheumatism with End-Results of Treatment (New York/London: Humphrey Milford/Oxford University Press, 1927), p 32.
71. Russell L Cecil & D Murray Angevine, "Clinical and experimental observations on focal infection, with an analysis of 200 cases of rheumatoid arthritis", Annals of Internal Medicine, 1938 Nov 1;12(5):577–584.
72. Fish E Wilfred (1939). "Bone infection". Journal of the American Dental Association. 26 (5): 691–712. doi:10.14219/jada.archive.1939.0156.
73. Louis I Grossman, Root Canal Therapy (Philadelphia: Lea & Febiger, 1940), ch 2, reprinted in Journal of Endodontics, 1982 Jan;8(Suppl):S18-S24, available at Robert Kaufmann's EndoExperience.com under "The endo file cabinet: Textbook excerpts: Grossman's Endodontics: Chapter on focal infection", Accessed online 17 Feb 2014.
74. Easlick KA (1951). "An evaluation of the effect of dental foci of infection on health". Journal of the American Dental Association. 42 (6): 615–97. PMID 14831976.
75. Philip M Preshaw & John J Taylor, ch 21 "Periodontal pathogenesis", in Michael G Newman, Henry Takei, Perry R Klokkevold & Fermin A Carranza, Carranza's Clinical Periodontology, 11th edn (St Louis: Saunders/Elsevier, 2012).
76. "Focal Infection". Journal of the American Medical Association. 150 (5): 490–1. 1952. doi:10.1001/jama.1952.03680050056016. PMID 14955464.
77. Joseph M Dougherty & Anthony J Lamberti, Textbook of Bacteriology, 3rd edn (St Louis: Mosby, 1954), p 231
78. Thomas C Galloway (1957). "Relation of tonsillectomy and adenoidectomy to poliomyelitis". Journal of the American Medical Association. 163 (7): 519–21. doi:10.1001/jama.1957.02970420001001. PMID 13398294.
79. Nikos Donos & Francesco D'Aiuto, ch 3 "Periodontitis: A modern clinical perspective", in Brian Henderson, Michael Curtis, Robert Seymour & Nikolaos Donos, eds, Periodontal Medicine and Systems Biology (West Sussex: Wiley-Blackwell, 2009), pp 33–34.
80. Ronald T Lewis, ch 25 "Soft tissue infection and loss of abdominal wall substance", in Robert Bendavid, ed, Abdominal Wall Hernias: Principles and Management (New York, Berlin, Heidelberg: Springer, 2001), p 192.
81. Technical Manual #8-225: Dental Specialist (Washington DC: Department of the Army Headquarters, 20 Sep 1971), pp Glossary-7 & 5-14.
82. John I Ingle, PDQ Endodontics, 2nd edn (Shelton CT: People's Medical Publishing House, 2009), p xv.
83. Michael Wilson, Rod McNab & Brian Henderson, Bacterial Disease Mechanisms: An Introduction to Cellular Microbiology (Cambridge: Cambridge University Press, 2002), p 597.
84. Hunter N (Oct 1977). "Focal infection in perspective". Oral Surgery, Oral Medicine, and Oral Pathology. 44 (4): 626–7. doi:10.1016/0030-4220(77)90308-5. PMID 269356.
85. Gerald J Domingue, Cell Wall-Deficient Bacteria: Basic Principles and Clinical Significance (Reading MA: Addison-Wesley Publishing, 1982), p 455.
86. Gendron Renée, Grenier Daniel, Maheu-Robert Léo-François (Jul 2000). "The oral cavity as a reservoir of bacterial pathogens for focal infections". Microbes and Infection. 2 (8): 897–906. doi:10.1016/S1286-4579(00)00391-9. PMID 10962273.{{cite journal}}: CS1 maint: multiple names: authors list (link)
87. Potgieter Marnie; Bester Janette; Kell Douglas B; Pretorius Etheresia (Jul 2015). "The dormant blood microbiome in chronic, inflammatory diseases". FEMS Microbiology Reviews. 39 (4): 567–591. doi:10.1093/femsre/fuv013. PMC 4487407. PMID 25940667.
88. Lida H Mattman, Cell Wall Deficient Forms: Stealth Pathogens, 3rd edn (Boca Raton FL: CRC Press, 2000), pp 286 & 289, while p 291 lists for p 289 a citation of Haley B (1996). "Root canal teeth contain toxins according to new and old research". Dent Amalgam Merc Synd. 6 (4): 1–4.
89. L H Mattman, Stealth Pathogens, 3rd edn (CRC Press, 2000), ch 1 "History".
90. Siqueira JF Jr, Rôças IN (2013). "As-yet-uncultivated oral bacteria: Breadth and association with oral and extra-oral diseases". Journal of Oral Microbiology. 5: 10.3402/jom.v5i0.21077. doi:10.3402/jom.v5i0.21077. PMC 3664057. PMID 23717756.
91. Li X, Kolltveit KM, Tronstad L, Olsen I (2000). "Systemic diseases caused by oral infection". Clinical Microbiology Reviews. 13 (4): 547–58. doi:10.1128/CMR.13.4.547-558.2000. PMC 88948. PMID 11023956.
92. Peter Mullany, Philip Warburton & Elaine Allan, ch 9 "The human oral metagenome", Karen E Nelson, ed, Metagenomics of the Human Body (New York, Dordrecht, Heidelberg, London: Springer, 2011), p 166.
93. Akshata KR, Ranganath A, Nichani V (2012). "Thesis, antithesis, and synthesis in periodontal and systemic interlink". Journal of Indian Society of Periodontology. 16 (2): 168–73. doi:10.4103/0972-124X.99257. PMC 3459494. PMID 23055580.
94. Otomo-Corgel J, Pucher JJ, Rethman MP, Reynolds MA (2012). "State of the science: Chronic periodontitis and systemic health". Journal of Evidence-based Dental Practice. 12 (3 Suppl): 20–8. doi:10.1016/S1532-3382(12)70006-4. PMID 23040337.
95. Atanasova Kalina R, Yilmaz Özlem (Jul 2015). "Prelude to oral microbes and chronic diseases: Past, present and future". Microbes and Infection. 17 (7): 473–83. doi:10.1016/j.micinf.2015.03.007. PMC 4485946. PMID 25813714.
96. Barnett Michael L (Oct 2006). "The oral-systemic disease connection: An update for the practicing dentist" (PDF). Journal of the American Dental Association. 137: 5S–6S. doi:10.14219/jada.archive.2006.0401. PMID 17012729.
97. Seymour GJ, Ford PJ, Cullinan MP, Leishman S, Yamazaki K (2007). "Relationship between periodontal infections and systemic disease". Clinical Microbiology and Infection. 13 (Suppl 4): 3–10. doi:10.1111/j.1469-0691.2007.01798.x. PMID 17716290.
98. Somma F, Castagnola R, Bollino D, Marigo L (2010). "Oral inflammatory process and general health. Part 1: The focal infection and the oral inflammatory lesion". European Review for Medical and Pharmacological Sciences. 14 (12): 1085–95. PMID 21375141.
99. Ford PJ, Yamazaki K, Seymour GJ (2007). "Cardiovascular and oral disease interactions: What is the evidence?". Primary Dental Care. 14 (2): 59–66. doi:10.1308/135576107780556806. PMID 17462139. S2CID 43371539.
100. Arigbede AO, Babatope BO, Bamidele MK (2012). "Periodontitis and systemic diseases: A literature review". Journal of Indian Society of Periodontology. 16 (4): 487–91. doi:10.4103/0972-124X.106878. PMC 3590713. PMID 23493942.
101. Examples: Ellen Hodgson Brown, Healing Joint Pain Naturally: Safe and Effective Ways to Treat Arthritis, Fibromyalgia, and Other Joint Diseases (New York: Broadway Books, 2001); Shirley MacLaine, Sage-ing While Age-ing (New York: Atria Books, 2007).
102. IAOMT, "IAOMT position paper on jawbone osteonecrosis", International Academy of Oral Medicine & Toxicology, 27 Jul 2014.
103. Stephen Barrett, "A critical look at cavitational osteopathosis, NICO, and 'biological dentistry'", Quackwatch, 4 Apr 2010.
104. Stephen Barrett, "Stay away from 'holistic' and 'biological' dentists", Quackwatch, accessed online: 17 Sep 2013.
105. J Craig Baumgartner, José F Siqueira Jr, Christine M Sedgley & Anil Kishen, ch 7 "Microbiology of endodontic disease", in John I Ingle, Leif K Bakland & J Craig Baumgartner, eds, Ingle's Endodontics, 6th edn (Hamilton Ontario: BC Decker, 2008), p 257: "Microorganisms found in failed endodontically treated teeth have either remained in the root canal from previous treatment or have entered since treatment via leakage. ... Those remaining from the original microbiota would need to have maintained viability throughout treatment procedures, including exposure to disinfectants, and thereafter adapted to a root canal environment in which the availability of a variety of nutrients is more limited because of lack of pulp tissue. This might occur as a result of an inability of chemomechanical instrumentation procedures to completely debride the root canal system in a single visit and because of the inaccessible locations of bacteria in isthmuses, accessory canals, and apical regions of canals. While it is considered that many such remaining bacteria will be unable to cause harm once entombed by the obturation material, there is little evidence for this".
106. Rogers AH (Aug 1976). "The oral cavity as a source of potential pathogens in focal infection". Oral Surgery, Oral Medicine, and Oral Pathology. 42 (2): 245–248. doi:10.1016/0030-4220(76)90131-6. PMID 1066607.
107. Ehrmann EH (Oct 1977). "Focal infection—the endodontic point of view". Oral Surgery, Oral Medicine, and Oral Pathology. 44 (4): 628–634. doi:10.1016/0030-4220(77)90309-7. PMID 269357.
108. J Craig Baumgartner, Leif K Bakland & Eugene I Sugita, ch 3 "Microbiology of endodontics and asepsis in endodontic practice" Archived 2011-08-16 at the Wayback Machine, in John Ide Ingle & Leif K Bakland, eds, Endodontics, 5th edn (Hamilton Ontario: BC Decker, 2002), p 64.
109. Thomas J Pallasch & Michael J Wahl (2003). "Focal infection: New age or ancient history?". Endodontic Topics. 4: 32–45. doi:10.1034/j.1601-1546.2003.00002.x. S2CID 9643840.
110. Mark A Breiner, Whole-Body Dentistry: A Complete Guide to Understanding the Impact of Dentistry on Total Health (Fairfield CT: Quantum Health Press, 2011), pp 171–174.
111. Mark A Breiner, Whole-Body Dentistry: A Complete Guide to Understanding the Impact of Dentistry on Total Health (Fairfield CT: Quantum Health Press, 2011), pp 168–69, 174–175.
112. Breiner, Whole-Body Dentistry (Quantum Health, 2011), pp
113. Mohammadi Zahed (Feb 2009). "Laser applications in endodontics: An update review". International Dental Journal. 59 (1): 35–46. PMID 19323310.
114. Ng YL, Mann V, Rahbaran S, Lewsey J, Gulabivala K (Dec 2007). "Outcome of primary root canal treatment: Systematic review of the literature—part 1. Effects of study characteristics on probability of success". International Endodontic Journal. 40 (12): 921–39. doi:10.1111/j.1365-2591.2007.01322.x. PMID 17931389.
115. Highlighting publications and findings by Price, by Meinig, and by Haley, holistic dentist Mark A Breiner advises not routine extraction of root-filled teeth, but routine monitoring, and extraction only when the tooth seems to especially impair health [Mark A Breiner, Whole-Body Dentistry: A Complete Guide to Understanding the Impact of Dentistry on Total Health (Fairfield CT: Quantum Health Press, 2011), pp 164, 168 & 175].