Among the broad spectrum of species of anaerobic bacteria in the normal flora of humans, a few exhibit marked pathogenic potential and are responsible for the majority of infections. The factors that determine the virulence of particular species are varied and probably interrelated. Just as most anaerobic infections are polymicrobial and depend on interactions of a combination of species, the virulence of a species probably depends on a combination of properties, including surface structures, metabolic functions, ability to avoid the host's defenses, and capacity to damage tissues. Thus, the production of each virulence factor--adhesins that attach to epithelial and red blood cells and to other bacteria, producing metabolically interdependent ecosystems; capsules that protect against phagocytosis and induce abscess formation; lipopolysaccharide; proteases, including those that degrade immunoglobulins and complement components; and other hydrolytic enzymes--represents only a component of virulence, but a consideration of these factors in combination begins to clarify the mechanisms by which anaerobes cause disease.