A yeast is a single-cell fungus. Many yeasts play a role in the production of foods such as bread or beer, but others can have an impact on human or animal health.

The term yeast originally refers to single-celled fungi capable of fermenting animal or plant matter. The name derives from the ability of the baker’s yeast Saccharomyces cerevisae to cause bread dough to rise. The term includes many single-celled fungal species, including the genus Candida, known to cause yeast infections in man.