Bacteriology, bak tihr ee OL uh jee, is the study of single-celled organisms called bacteria. Some kinds of bacteria cause serious diseases, such as leprosy and tuberculosis. But many kinds are helpful. For example, some bacteria keep our environment clean by helping dead organisms decay. Bacteriologists study harmful bacteria to determine the exact way they cause disease so that better vaccines can be developed. They study helpful bacteria to learn to use and control them.