The microbiological flora of the lower female genital tract provides a dynamic, complex example of microbial colonization, the regulation of which is not fully understood. When an exogenous bacterial species, with its array of virulence factors, is introduced into the host, disease does not always occur. Conversely, under selected conditions, commensal endogenous bacteria—for example, Gardnerella vaginalis and group B streptococci—can participate in disease processes. Disease caused by both exogenous and endogenous bacteria correlates positively with a markedly increased level of bacterial replication. The key question is what determines the quantity of a given bacterium at any given time. For disease to occur, exogenous or endogenous bacteria that possess pathogenic prerequisites must attain replicative dominance. Their ability to do so is potentially governed by inhibitory or synergistic interrelationships with other microbes.