Anthrax is an infectious disease caused by the bacterium Bacillus anthracis, which can form spores that can survive in the environment for long periods of time. Anthrax infection can occur in three forms: cutaneous (skin), inhalation (lung), and gastrointestinal (stomach and intestines). Humans usually contract the disease when handling products from contaminated animals.
If people have intentionally been exposed, as in a bioterrorist release, breathing in spores is the most likely route of exposure that might lead to a serious infection (inhalation anthrax).