An anaerobic infection is caused by bacteria which cannot grow in the presence of oxygen. These bacteria are called anaerobic bacteria or anaerobes. Anaerobic bacteria can infect deep wounds, deep tissues, and internal organs where there is little oxygen. These infections are characterized by abscess formation, foul-smelling pus, and tissue destruction.

Anaerobic bacteria grow in places which completely, or almost completely, lack oxygen. They are normally found in the mouth, gastrointestinal tract, and vagina, and on the skin. Anaerobic bacteria can cause an infection when a normal barrier, such as skin, gums, or intestinal wall, is damaged due to surgery, injury, or disease. Usually, the immune system kills any invading bacteria, but sometimes the bacteria are able to grow and cause an infection.

Commonly known diseases caused by anaerobic bacteria include gas gangrene, tetanus, and botulism. Nearly all dental infections are caused by anaerobic bacteria.

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