The human fetus is free of microorganisms.1 After initial exposure at birth, most organisms are soon eliminated, but others become permanently established and the dynamic process of colonization begins. The adult body harbors a dense, diverse, indigenous flora that includes bacteria, viruses, fungi and protozoa. Interaction between these various microbial ecosystems determines the normal flora. Microorganisms of the normal flora establish symbiotic relationships (mutualism, commensalisms, or parasitism) with their human host and each other.2,3

Factors that modify or shift the balanced environment of the normal flora (age, altered anatomy, diet, local and systemic conditions, or pharmacotherapy) may predispose an individual to infection.4,5 Infection, the invasion and multiplication of microorganisms in body tissues, results in cellular injury due to competitive metabolism, toxin production, or immune-mediated reactions. An infection may be autogenous, caused by the body’s normal flora; or it may be a cross-infection, related to the proliferation of transient organisms.6