Anaerobic bacteria are bacteria that develop and live in areas that are absent from the presence of oxygen. They typically will infect areas of the body that are internal and normally sterile, or free from the presence of oxygen. As to the development and evolution of such bacteria, there is an evolutionary process by which certain strains of bacteria did not need oxygen to conduct their life processes. These bacteria continued to thrive in oxygen-poor areas that would normally end the life-cycle of aerobic bacteria. The development and cultivation of a specific trait, such as the lack of a need for oxygen, is the result of natural selection of those bacteria best suited to exist in the conditions specified.