Bacteria are simple, one-celled organisms. They are not visible to the eye.
Some bacteria are useful and some are harmful.
Examples for bacterial hazards are tuberculosis, tetanus, salmonellosis. Tuberculosis, is a disease caused by a bacteria called Mycobacterium tuberculosis. The bacteria can attack any part of the body, but they usually attack the lungs. TB disease was once the leading cause of death in this country. As a result, TB slowly began to disappear in the United States.