Colonization is the presence of a microorganism in or on a host, with growth and multiplication but without any overt clinical expression or detected immune response in the host at the time it is isolated. Normal colonization in humans begins during the birth process and through subsequent contacts with the inanimate or animate environments until a delicately balanced "normal" flora is established; subsequently, the precise components of this flora evolve. This normal flora, such as coagulase-negative Staphylococcus or Staphylococcus aureus on the skin or Candida albicans in the gastrointestinal tract, vagina, or perineal area, can result in infection when normal body defenses are impaired through underlying disease, immunomodulating therapy, or the use of invasive devices, or when the delicate balance of the normal flora is altered through antimicrobial therapy. Many, if not most, hospital-acquired infections result directly or indirectly from patient colonization; studies have shown that hospitalized patients are colonized rapidly with hospital flora. Recognizing this reservoir of patients colonized with epidemiologically important pathogens in our hospitals and improving barrier precaution measures or altering host susceptibility will be necessary if we are to reduce the incidence of infections with these organisms.