Our understanding of infectious diseases continues to expand rapidly,
and has led to the realization that microorganisms are responsible for
, or at least contribute to, numerous diseases that were never before
associated with infectious etiologies. However, a review of medical hi
story reminds us that this is not so novel, an idea. Not long after th
e widespread acceptance of bacteriology and the germ theory and with a
n increased awareness of public hygiene, there was a period during whi
ch it seemed that nearly all diseases would prove to be the result of
infections. One popular proposal that championed such an idea was the
theory of focal infection. This article reviews this theory by conside
ring the key concepts and developments that likely inspired it, and ex
amines the work of the theory's most visible proponent, Dr. Frank Bill
ings.