Medical mycology addresses fungal infections in humans. Yeasts are unicellular fungi that reproduce by budding. The most clinically significant yeasts belong to the genera Candida and Cryptococcus. The incidence of invasive yeast infections has increased dramatically with the growing population of immunocompromised patients.

Candida species are commensals of the gastrointestinal tract, skin, and mucous membranes. C. albicans is the most common pathogenic species, but non-albicans species including C. glabrata, C. parapsilosis, C. tropicalis, and C. krusei are increasingly important. Candidiasis ranges from superficial mucocutaneous infections (oral thrush, vulvovaginal candidiasis) to invasive candidemia with dissemination to deep organs. Predisposing factors include antibiotic use, immunosuppression, indwelling catheters, and diabetes.

Laboratory diagnosis of candidiasis includes direct microscopy of clinical specimens showing budding yeast cells with pseudohyphae, culture on Sabouraud dextrose agar, and identification by germ tube test (C. albicans is positive), chromogenic agar, or MALDI-TOF MS. Antifungal susceptibility testing by broth microdilution guides treatment selection.

Cryptococcus neoformans and C. gattii cause cryptococcosis. C. neoformans is a major opportunistic pathogen in HIV/AIDS patients, causing meningoencephalitis. The fungus has a polysaccharide capsule that is its primary virulence factor. Diagnosis uses India ink staining of CSF specimens to visualize encapsulated yeast cells, antigen detection by latex agglutination or lateral flow assay, and culture. Treatment involves antifungal agents such as amphotericin B with flucytosine for induction therapy followed by fluconazole maintenance.

Antifungal resistance is a growing concern. Azole resistance in Candida species, particularly C. glabrata and C. auris, limits treatment options. Echinocandin resistance is emerging. Susceptibility testing is essential for guiding therapy in invasive infections.