Disease-producing, or pathogenic, bacteria are parasites of living organisms. Bacteria enter humans in a variety of ways. Airborne bacteria enter through abrasions in the skin or by passing directly into the nose, throat, or lungs. They cause such diseases as diphtheria, pertussis (whooping cough), staph infections, strep throat, tetanus, and tuberculosis. Other bacteria enter the body through contaminated food or water. They cause such diseases as typhoid fever, dysentery, cholera, and botulism. Some strains of bacteria secrete toxins (poisons) into the bloodstream, which carries them to other parts of the body. Some bacterial diseases can be prevented by routine immunizations during childhood. If the diseases are contracted, they are usually treated with antibiotics, such as penicillin, and with sulfa drugs. Persons who are especially susceptible to bacterial infections, such as those recovering from surgery or those undergoing chemotherapy, are routinely given antibiotics as a preventive measure.
Bacteria also cause disease in plants and in other animals. In plants, they infect the roots, leaves, or stems, causing such diseases as rots, blights, or wilts. Bacterial diseases of livestock include anthrax and Bang's disease.