Yeasts are single-celled fungi. Yeast species inhabit diverse habitats, including skin, marine water, leaves, and flowers.

Some yeast are beneficial, being used to produce bread or allow the fermentation of sugars to ethanol that occurs during beer and wine production (e.g., Saccharomyces cerevisiae). Other species of yeasts are detrimental to human health. An example is Candida albicans, the cause of vaginal infections, diaper rash in infants, and thrush in the mouth and throat. The latter infection is fairly common in those whose immune system is compromised by another infection such as acquired immunodeficiency syndrome.

The economic benefits of yeast have been known for centuries. Saccharomyces carlsbergensis, the yeast used in the production of various types of beer that result from "bottom fermentation," was isolated in 1888 by Dr. Christian Hansen at the Carlsberg Brewery in Copenhagen. During fermentation, some species of yeast are active at the top of...