破傷風菌
If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus (caused by "Clostridium tetani"), typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy (caused by "Micobacterium leprae)" and tuberculosis (caused by "Mycobacterium tuberculosis"). A pathogenic cause for a known medical disease may only be discovered many years later, as was the case with "Helicobacter pylori" and peptic ulcer disease. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight and wilts in plants, as well as Johne's disease, mastitis, salmonella and anthrax in farm animals.