Anthrax vaccine in a bottle with a syringe. Anthrax is caused by Bacillus anthracis, a bacterium that produces spores and is most commonly found in wild and domestic mammals. Anthrax occurs in humans when they are exposed to infected animals, infected animal tissue, or directly to B. anthracis. Anthrax can occur in three forms: cutaneous (from skin contact), inhalational (airborne), and gastrointestinal (via ingested food). The vaccine is made from inactive filtrates of Anthrax, which stimulates the immune system to produce antibodies without causing an infection. The vaccine is used mostly by USA military units to protect them against possible bioterrorist attacks abroad.