A yeast is a single-celled fungus. It is related to other fungi that people are more familiar with. These include edible mushrooms which are available at the local supermarket, common baker’s yeast used to leaven bread, molds that ripen blue cheese as well as molds that produce antibiotics for medical and veterinary use. Many consider edible yeast and fungi to be as natural as fruits and vegetables.
Over 600 different species of yeast are known. They are found in association with other microorganisms as part of the normal inhabitants of soil, vegetation, marine and other aqueous environments. Some yeast species are also natural inhabitants of man and animals. While some species are highly specialized and found only in certain habitats at certain times of the year, other species are generalists and can be isolated from many different sources.
Baker’s (or brewer's) yeast (Saccharomyces cerevisiae) is used to leaven bread throughout the world and it is the type of yeast that people are most familiar with. It is also used in making beer and wine, although sometimes other closely related species such as Saccharomyces carlsbergensis or Saccharomyces bayanus are used instead.
A typical yeast cell is approximately equal in size to a human red blood cell and is spherical to ellipsoidal in shape. Because it is so very tiny, it takes about 30 billion yeast cells to make up to one gram of compressed baker’s yeast. Yeasts reproduce by budding, a process during which a new bud grows from the side of the existing cell wall. This bud eventually breaks away from the mother cell to form a separate daughter cell. Each budding yeast cell, on average, undergoes this budding process 12 to 15 times before it is no longer capable of budding.
During commercial production, yeast is grown under carefully controlled conditions on a sugar containing medium typically composed of beet and cane molasses. Under ideal growth conditions a yeast cell reproduces every two to three hours. Under anaerobic (devoid of oxygen) conditions the yeast cell will carry out the metabolic process of fermentation to produce ethanol and carbon dioxide.
Other notable yeasts
Schizosacharomyces pombe is a yeast first isolated from East African millet beer (pombe is the Swahili word for beer). However, it is most important as a laboratory experimental organism for the investigation of cell biology. Sir Paul Nurse won the 2001 Nobel Prize for Physiology and Medicine for his discoveries of how the cell division cycle is regulated in S. pombe. 
Candida albicans is a common commensal yeast which lives on human skin and in the gut. About 50% of people carry it, with no noticeable effect. It is most well known as the cause of thrush, although less frequently it can cause nasty oral infections. In severely immunocompromised patients it can spread through the body as hyphae and invade vital organs, with a high mortality rate.  The switch between harmless single cells and pathogenic hyphae is regulated by the numbers of yeast cells in a particular environment, in a process known as quorum sensing.