What are the harmful bacteria?

There is a wide variety of bacteria. Some of them are beneficial to humans, and some are a great threat. For example, E. coli, Koch’s bacillus, staphylococci and various other bacteria are dangerous for humans. But in nature, they are all very important, since even bacteria harmful to us help other organisms to evolve. They promote natural selection and so on. But you need to remember about the ways of transmission of diseases in order to prevent diseases and their further spread.