Different kinds of bacteria have a tendency to affect different organs and systems of the body, producing infectious diseases, each with its own group of symptoms or signs. Understanding the differences in bacteria and the methods used to study them aid in providing diagnostic information that help with not only the selection of appropriate antibiotics for medical treatment, but which antiseptic agents or disinfectants to use.

It is important to realize that the majority of bacteria coexist peacefully with the body. Some bacteria are helpful, such as those existing in the intestine that feed on other microscopic organisms that might actually harm the body. Some bacteria can also produce vitamins used by the body such as vitamin C and vitamin K.