YEAST

Yeast, a microscopic, one-celled organism belonging to the group of organisms called fungi. There are many kinds of yeasts, some of them of great importance to humans. 

Yeast is necessary to make leavened bread, beer, cheese, wine, and whiskey. It is rich in B vitamins; a form of yeast called brewer's yeast is used as a diet supplement. Yeast is also used in genetic engineering to produce large quantities of certain hormones and enzymes, which are used for such medical purposes as healing wounds and reducing inflammation. Some types of yeast, however, cause disease; candidiasis, a skin infection, is an example.
Yeasts are found in the soil, in water, on the surface of plants, and on the skin of humans and other animals. Like other fungi, yeasts obtain food from the organic matter around them; they secrete enzymes that break down the organic matter into nutrients they can absorb.
The yeast cell is oval or round and has a thin membrane. Under ideal conditions of moisture, temperature, and food supply, it reproduces asexually, by budding. When a yeast cell reaches full growth, a budlike swelling forms on its surface. Part of the parent cell's nucleus goes into this bud, and a wall is formed between the parent cell and the bud, which then becomes a separate cell. This new cell may break off when it is full grown. It may, however, remain attached as it produces another bud. In this way, chains or clusters of cells are formed. Budding is a rapid process, requiring about 20 minutes to produce a new organism.

http://science.howstuffworks.com/life/fungi/yeast-info.htm