Terminology and Microbiology
Traditionally, fungal infections have been classified into specific categories based on both anatomic location and epidemiology. The most common general anatomic categories are mucocutaneous and deep organ infection; the most common general epidemiologic categories are endemic and opportunistic. Although mucocutaneous infections can cause serious morbidity, they are rarely fatal. Deep organ infections also cause severe illness in many cases and, in contrast to mucocutaneous infections, are often fatal. The endemic mycoses (e.g., coccidioidomycosis) are infections caused by fungal organisms that are not part of the normal human microbial flora and are acquired from environmental sources. In contrast, opportunistic mycoses are caused by organisms (e.g., Candida and Aspergillus) that commonly are components of the normal human flora and whose ubiquity in nature renders them easily acquired by the immunocompromised host. Opportunistic fungi cause serious infections when the immunologic response of the host becomes ineffective, allowing the organisms to transition from harmless commensals to invasive pathogens. Frequently, the diminished effectiveness of the immune system is a result of advanced modern therapies that coincidentally either unbalance the host's microflora or directly interfere with immunologic responses. Endemic mycoses cause more severe illness in immunocompromised patients than in immunocompetent individuals.
Patients acquire deep organ infection with endemic fungi almost exclusively by inhalation. Cutaneous infections result either from hematogenous dissemination or, more often, from direct contact with soil—the natural reservoir for the vast majority of endemic mycoses. The dermatophytic fungi may be acquired by human-to-human transmission, but the majority of infections result from environmental contact. In contrast, the opportunistic fungus Candida invades the host from normal sites of colonization, usually the mucous membranes of the gastrointestinal tract. In general, innate immunity is the primary defense mechanism against fungi. Although antibodies are formed during many fungal infections (and even during commensalism), they generally do not constitute the primary mode of defense. Nevertheless, in selected infections, as discussed below, measurement of antibody titers may be a useful diagnostic test.
Three other terms frequently used in clinical discussions of fungal infections are yeast, mold, and dimorphic fungus. Yeasts are seen as rounded single cells or as budding organisms. Candida and Cryptococcus are traditionally classified as yeasts. Molds grow as filamentous forms called hyphae both at room temperature and in invaded tissue. Aspergillus, Rhizopus [the species that causes mucormycosis (zygomycosis)], and fungi commonly infecting the skin to cause ringworm and related cutaneous conditions are classified as molds. Variations occur within this classification of yeasts and molds. For instance, when Candida infects tissue, both yeasts and filamentous forms may occur (except with C. glabrata, which forms only yeasts in tissue); in contrast, Cryptococcus exists only in yeast form. Dimorphic is the term used to describe fungi that grow as yeasts or large spherical structures in tissue but as filamentous forms at room temperature in the environment. Classified in this group are the organisms causing blastomycosis, paracoccidioidomycosis, coccidioidomycosis, histoplasmosis, and ...