The focal infection theory: appraisal and reappraisal.

This paper discusses the past, present, and future of the focal infection theory of disease. A focal infection is a localized or general infection caused by the dissemination of microorganisms or toxic products from a focus of infection. The resurgence of the focal infection theory of disease has been greeted with great enthusiasm in some quarters; however, the present evidence for the relationship of oral microorganisms and systemic disease is very limited due not only to a dearth of prospective studies and a complete lack of interventional studies but also to very significant methodological difficulties associated with the clinical studies that have been performed.