Most cases of yeast infection in the United States are caused by Candida albicans . There are about 500 species of yeasts in 60 genera, or about 1000 species of yeasts or yeast-like organisms. Some fungi are called yeast-like because they exist as a yeast for part of their life cycle, but can be hyphal for a significant portion of it. The terms yeast and yeastlike are vernacular terms for unicellular fungal organisms that reproduce by budding. It is useful only to describe a morphological form of a fungus.
Most yeasts have affinities to Ascomycota, but a small percentage have affinities to Basidiomycota.
Some alteration of the host's cellular defenses, physiology, or normal flora must take place before colonization, infection, and disease production can take place. Most of these have airborne spores or conidia and can be isolated as contaminants from skin, sputum, feces or other clinical specimens. This can lead to confusion about which organism is actually the pathogen. Only a few species in a few genera have regularly been associated with production of disease in humans or animals. In compromised host there are many others that can be opportunists.