Focal infection theory is the historical concept that many chronic diseases, including systemic and common ones, are caused by focal infections. A focal infection is a localized infection, often asymptomatic, that causes disease elsewhere in the host, but the present medical consensus is that focal infections are fairly infrequent and mostly limited to fairly uncommon diseases.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). (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.)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. Historical focal infection theory, rather, so explained virtually all diseases, including arthritis, atherosclerosis, cancer, and mental illnesses.Paul R Stillman & John O McCall, A Textbook of Clinical Periodontia, (New York: Macmillan Co, 1922), "ch 18 Focal infection".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.
An ancient concept that took modern form around 1900, focal infection theory was widely accepted in medicine by the 1920s. In the theory, the focus of infection might lead to secondary infections at sites particularly susceptible to such microbial species or toxin. 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. 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.
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. Untreated endodontic disease retained mainstream recognition as fostering systemic disease.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.Gavett G, "Tragic results when dental care is out of reach"4, PBS Frontline website, 26 Jun 2012 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.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". 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.
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.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. 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.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. 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.
Rise and popularity (1890s–1930s)
Roots and dawn
Germ theory Hippocrates, in ancient Greece, had reported cure of an arthritis case by tooth extraction. Yet focal infection, as such, appeared in modern medicine in 1877, when Karl Weigert reported "dissemination of 'tuberculosis poison' ". 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 speciesIn 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.—whereby Koch announced discovery of the "tubercle bacillus" in 1882, fully premising the modern principle of focal infection. 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.Barry D. Silverman, "William Henry Welch (1850–1934): The road to Johns Hopkins", Proc (Bayl Univ Med Cent), 2011 Jul;24(3):236–242.
As progressively more diseases drew an infectious hypothesis that led to a pathogen discovery, conjectures grew that virtually all diseases are infectious.R V Gibbons, "Germs, Dr. Billings, and the theory of focal infection", Clin Infect Dis, 1998 Sep;27(3):627–633. 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.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). In 1894, Miller became the first to identify bacteria in samples of tooth pulp.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. Miller advised root canal therapy. 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".Andrew Scull, Madhouse: A Tragic Tale of Megalomania and Modern Medicine (New Haven: Yale University Press, 2005), pp 33–37. Around 1900, British surgeons, still knife-happy, were urging "surgical bacteriology".
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.Caroline Barranco, "The first live attenuated vaccines", Nature Portfolio, Springer Nature Limited, 28 Sep 2020. Its success funded Pasteur's formation of the globe's first biomedical research institute, the Pasteur Institute. 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.Alfred I Tauber & Leon Chernyak, Metchnikoff and the Origins of Immunology: From Metaphor to Theory (New York: Oxford University Press, 1991), pp viii, 11. Later the institute's director and a 1908 Nobelist, Metchnikoff believed, as did his German immunology rival Paul Ehrlich—theorist on antibody, mediating acquired immunityImmunology'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.—and as did Pasteur, too, that nutrition influences immunity. 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. Metchnikoff reasoned that the colon functions as a "vesitigal cesspool" that stores waste but is unneeded.Philip A. Mackowiak, "Recycling Metchnikoff: Probiotics, the intestinal microbiome and the quest for long life", Front Public Health, 2013 Nov 13;1:52.
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". 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. 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. 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". But in the 1910s, as North American medical schools emulated the German model—that is, "scientific medicine"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.—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.
Medical popularity
Hunter on "oral sepsis"