Many species of fungi and algae may cause disease among the increasing population of individuals at risk. These microorganisms, loosely called opportunistic agents, cannot cause disease unless two major criteria are met: (1) the patient suffers from some predisposing factor that has mechanically (eg, trauma) or immunologically (eg, organ transplantation) decreased the capacity to resist infection and (2) the infecting agent can survive and multiply at body temperature (37°C [98.6°F]). At present, the number of these opportunistic agents reported to cause infection exceeds 200.

Although some opportunistic fungal infections are noteworthy for specific predisposing factors (eg, ketoacidosis, zygomycete infection), neutropenia or a defect in cell-mediated immunity is the usual predisposing factor. However, any trauma, disease state, or pharmacologic insult to host defenses increases the chance of fungal invasion, even from a patient's own normal flora.

The microorganisms considered in this chapter (Table 179.1) are ubiquitous but are uncommon causes of disease in humans. Therefore the diagnosis is usually made when a patient has an infectious disease that does not respond to antibacterial therapy, when the microbiology laboratory reports the isolation of one of these agents, or when the pathologist identifies a fungus or alga on histopathology.