Focal infection theory

In focal infection theory (FIT), a localized infection, typically obscure, disseminates microorganisms or their toxins elsewhere within the individual's own body and thereby injuries distant sites, where ensuing dysfunction yields clinical signs and symptoms and eventually disease, perhaps systemic and usually chronic, such as arthritis, atherosclerosis, cancer, or mental illness. (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 identified and invasion progresses contiguously, as in gangrene.) This ancient concept took modern form around 1900, and was widely accepted in Anglosphere medicine by the 1920s.In the theory, the focus of infection is often unrecognized, while secondary infections might occur at sites particularly susceptible to such microbial species or toxin. Several locations were commonly claimed as foci—appendix, urinary bladder, gall bladder, kidney, liver, prostate, and nasal sinus—but most commonly oral tissues. Not only chronically infected tonsils and dental decay, but also sites of dental restoration and root canal therapy were indicted as the 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 chronic diseases.Drawing severe criticism in the 1930s, focal infection theory, whose popularity zealously exceeded consensus evidence, was generally discarded in the 1940s amid overwhelming consensus of its general falsity, whereupon dental restorations and root canal therapy became again favored. Untreated endodontic disease retained recognition as fostering systemic disease, but only alternative medicine and later biological dentistry continued highlighting sites of dental treatment—root canal therapy, dental implant, and, as newly claimed, tooth extraction, too—as foci of infection promoting systemic diseases. 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 infection theory 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. 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 hypotheses of stealth infections via dental treatment. Despite some doubts renewed in the 1990s by critics of conventional dentistry, dentistry scholars maintain that endodontic therapy can be performed without creating focal infections.