Yeast: A group of single-celled fungi that reproduce by budding.
Most yeast are harmless (some are used in baking and brewing). Yeast is commonly present on normal human skin and in areas of moisture, such as the mouth and vagina, usually without causing any problems.
However, yeast can produce disease in people. For example, the yeast Candida (once called Monilia) causes thrush (oral infection) and diaper rash in infants, fingernail infections, vaginal area infections in women after puberty, and a host of problems in patients with immune deficiency.