Yeast are single-cell fungi with ovoid or spherical shapes that are grouped according to the cell-division process into budding yeast (e.g., the species and strains of Saccharomyces cerevisiae and Blastomyces dermatitidis), or fission yeast (e.g., Schizosaccharomyces) species.

Yeast species are present in virtually all natural environments such as fresh and marine water, soil, plants, animals, and in houses, hospitals, schools, etc. Some species are symbiotic, while others are parasitic. Parasitic species may be pathogenic (i.e., cause disease) either because of the toxins they release in the host organism or due to the direct destruction of living tissues such as skin, internal mucosa of the mouth, lungs, gastrointestinal, genital and urinary tracts of animals, along with plant flowers, fruits, seeds, and leaves. They are also involved in the deterioration and contamination of stored grains and processed foods.

Yeast and other fungal infections may be...