Bacteria, (singular: Bacterium), microscopic, one-celled organisms. Most bacteria range in size from 0.15 to 4 micrometers. (A micrometer is 0.001 millimeter [about 1/25,000 of an inch].) The largest known bacterium, Thiomargarita namibiensis, is more than 750 micrometers long. Bacteria are found in soil, in water, and in the air. They are also found on plants, on the surfaces of decayed or decaying matter, and in the intestinal and reproductive tracts of humans and other animals.

Although many infectious diseases are caused by bacteria, there are relatively few disease-producing species compared to the large number of harmless and useful ones.