Opportunistic Infection: A Review

Infectious diseases caused by microorganisms belonging to the normal flora of a host and generally regarded as nonpathogenic are designated opportunistic infections. This term also implies some derangement in normal host defensive mechanisms. Once considered rare, these infections arc being encountered with increasing frequeney, especially in medical centers where complex diagnostic, operative and therapeutic procedures are employed. A sizable population now exists whose survival, though prolonged by medical science, is constantly threatened by this type of infectious disease. Opportunistic infections do not occur in a haphazard manner; the typical circumstances during which they occur are discussed to demonstrate that it should be possible to anticipate their development, establish a precise diagnosis and arrive at a rational plan of medical management. Representative bacterial, fungal, viral and parasitic opportunists and the wide spectrum of diseases caused by these organisms are discussed to illustrate the multiplicity of loci they may infect and the variety of clinical pictures with which they may present. The concept that deficiencies in host resistance confer the potential for pathogenicity to virtually every microorganism is emphasized.