Bacteria are unicellular prokaryotes and visible under a compound microscope. Though many bacteria are harmless,some are parasitic and produce diseases. Bacteria can enter the host body through the mouth, nostrils, cuts and bruises on the skin. They multiply rapidly, producing toxins in high concentration to affect health. Some bacterial diseases in man are Tuberculosis, Leprosy, Cholera, Typhoid, Diphtheria, Pertusis, Tetanus, Plague, Pneumonia, Syphilis, Gonorrhoea, etc.