Focal infection theory

Focal infection theory (FIT) explains that a localized infection, often asymptomatic, can disseminate microorganisms or their toxins to distant sites within one's own body, and thereby initiate disease. The resulting diseases are often systemic and chronic, but usually not thought infectious, 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 identifiable 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.