Focal infection theory (FIT) is the idea that a local infection affecting a small area of the body can lead to subsequent infections or symptoms in other parts of the body due either to the spread of the infectious agent itself or toxins produced from it.
Early proponents of FIT variously listed infection in the tonsils, oral cavity, sinuses, prostate, appendix, bladder, gall bladder, and kidney as possible causes of systemic disease. Many of these areas were targets of operations in an attempt to heal the entire body. Opponents at the time criticized FIT as blaming systemic problems on "anything that is readily accessible to surgery."
The theory became popular in the late 19th and early 20th century, especially in the field of oral medicine. Connecting disease to dental infections resulted in an extremely high number of pulled teeth during the 1920s. Studies in the 1930s showed that previous evidence in support of this application were flawed, and more rigorous research did not find evidence for dental infections as a major source of systemic disease.
FIT was greatly refined over the 20th century to explain specific diseases such as tuberculosis, gonorrhea, syphilis, pneumonia, typhoid fever, and mumps, all of which do follow its general principle albeit in much narrower pathways than was originally suggested by the theory's early proponents.
In the dental community, current consensus is that FIT is not a valid reason to remove teeth or to avoid root canals—as ways to cure systemic infections or prevent bacteria from spreading from the tooth to the body. More recent studies have shown a relationship between dental health and heart disease, but these are only correlational and do not support conclusions about the root cause. At the same time the academic community has cautiously reinvestigated FIT, the natural health and holistic dentist communities have enthusiastically revived early research into FIT as the basis for a broad criticism of mainstream dental practices. The academic community considers these critiques unsupported by both the early and more recent research into FIT.
Read more about Focal Infection Theory: Primary Cause of Systemic Disease Era (1890s—1950s), Evolution and Revival of FIT
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... However, the fall of focal infection theory as primary cause of systemic disease did not mean the theory itself did not continue to find enough supporters to appear ... All these factors have resulted in a disagreement not only about when focal infection theory fell out of favor but also the degree to which it did ... "(i)n the 1930s, editorials and research refuted the theory of focal infection" ...