Anthrax, AN thraks, is a serious infectious disease that chiefly affects animals but can also occur in people. It is caused by the bacterium Bacillus anthracis. Anthrax usually affects plant-eating animals infected by eating anthrax spores (inactive bacteria) from the soil. Anthrax spores can survive harsh conditions and occur in soil throughout the world, including the United States. People can get anthrax through contact with infected animals or contaminated animal products. But, naturally occurring anthrax is rare in human beings today.