Bacteria comprise an extremely diverse group of organisms that are ubiquitous. Although the majority of bacterial species do not overtly cause disease, bacteria as a whole have the infamous association with illness, ranging from a simple rash to death.

Other forms of bacteria that are less well known by the public are the relatively less virulent ones. The activity of some bacteria benefit both the bacteria and their human host. Certain strains of Escherischia coli and Bacteroides fragilis, for example, flourish in the human intestine. Without these organisms, there is a possibility of contracting illness from more serious forms of bacteria (e.g., Clostridium difficile).

There are other species that neither benefit the host nor harm it either. Although these bacteria do not pose a concern in general, the bacteria may cause disease in a sub-type of the population. Immunocompromised or hospitalized patients are particularly susceptible to bacteria that would otherwise not cause disease. These phenomena are termed opportunistic and nosocomial infections.