Anthrax is a serious disease caused by the bacterium Bacillus anthracis. While the disease most commonly occurs in animals (such as cattle, sheep, and antelopes), it can affect humans. People may develop anthrax if they are exposed to infected animals, to tissue from infected animals, or to anthrax spores used as a bioterrorist weapon. The three major types of anthrax include: cutaneous (skin) anthrax, inhalation anthrax, and gastrointestinal anthrax. Teatment usually involves antibiotics and providing relief of symptoms and complications as the body fights the anthrax bacteria.