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Mycology is the study of fungi, which are eukaryotic organisms that evolved in tandem with the animal kingdom. However, unlike animals, most fungi are nonmotile and possess a rigid cell wall. Unlike plants, fungi are nonphotosynthetic. Approximately 80,000 species of fungi have been described, but fewer than 400 are medically important, and less than 50 species cause more than 90% of the fungal infections of humans and other animals. Rather, most species of fungi are beneficial to humankind. They reside in nature and are essential in breaking down and recycling organic matter. Some fungi greatly enhance our quality of life by contributing to the production of food and spirits, including cheese, bread, and beer. Other fungi have served medicine by providing useful bioactive secondary metabolites such as antibiotics (eg, penicillin) and immunosuppressive drugs (eg, cyclosporine). Fungi have been exploited by geneticists and molecular biologists as model systems for the investigation of a variety of eukaryotic processes, including molecular and cellular biology and development. Overall, fungi exert their greatest economic impact as phytopathogens; the agricultural industry sustains huge crop losses every year as a result of fungal diseases of rice, corn, grains, and other plants.

Like all eukaryotes, each fungal cell has at least one nucleus with a nuclear membrane, endoplasmic reticulum, mitochondria, and secretory apparatus. Most fungi are obligate or facultative aerobes. They are chemotrophic, secreting enzymes that degrade a wide variety of organic substrates into soluble nutrients which are then passively absorbed or taken into the cell by active transport.

Fungal infections are mycoses. Most pathogenic fungi are exogenous, their natural habitats being water, soil, and organic debris. The mycoses with the highest incidence—candidiasis and dermatophytosis—are caused by fungi that are part of the normal human microbiota and highly adapted to survival on the human host. For convenience, mycoses may be classified as superficial, cutaneous, subcutaneous, or systemic, invading the internal organs (Table 45-1). The systemic mycoses may be caused by endemic fungi, which are usually primary pathogens, or by ubiquitous, often secondary opportunistic pathogens. Grouping mycoses in these categories reflects their most common portal of entry and initial site of involvement. However, there is considerable overlap, since systemic mycoses often exhibit subcutaneous manifestations and vice versa. Most patients who develop opportunistic infections have serious underlying diseases and compromised host defenses. But primary systemic mycoses also occur in such patients, and the opportunists often infect immunocompetent individuals. During infection, most patients develop significant cellular and humoral immune responses to the fungal antigens.

Table 45-1 The Major Mycoses and Causative Fungi

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