In nature, anthrax is rare and typically only strikes grazing animals, such as cows and sheep. The anthrax bacterium can live in the soil for decades as a tough spore. Ingested, inhaled, or after entering a wound, spores can come to life—germinating into metabolically active bacteria, reproducing, and releasing deadly toxins. When an animal dies from anthrax and begins to rot, the bacteria are exposed to the air, which stimulates them to develop into tough spores again.

Humans rarely get anthrax (also called woolsorter's disease), but when they do, it is usually through occupational exposure to infected animals or animal products. Terrorists, however, have also exposed humans to these spores, increasing the number of anthrax cases in the human population.