By definition, dental caries is an infectious and transmissible disease because it is caused by bacteria colonizing the tooth surfaces. Unlike most infectious diseases affecting humans, caries is the result of an imbalance of the indigenous oral biota rather than a nonindigenous, exogenous pathogen. The introduction of refined sugar into modern society's diet has tipped the balance from health to disease. New insight into the natural history of the leading cariogenic bacteria, the mutans streptococci, may contribute ways to control or prevent this infectious disease. Here, we use the host-parasite model as a platform for viewing the pathogenicity of the caries process in contrast to other infectious diseases.