Yeasts are unicellular—single cells—throughout their entire life. About 600 different species of yeast have been discovered. Only a few of them cause disease in plants, animals, and humans; a few are used commercially; and the majority live in the soil, on plant surfaces, and decaying organic matter. The average yeast cell is 4 to 12 micrometers (MGRm) long, but with all the different species, this is merely an estimate.