Anthrax is caused by Bacillus anthracis bacteria and can infect humans through the skin, lungs, or digestive system. It commonly affects farm animals. Skin anthrax causes a painless sore or ulcer, while inhalation anthrax results in flu-like symptoms. Digestive anthrax causes abdominal pain, diarrhea, and vomiting. All forms can be treated with antibiotics such as penicillin, doxycycline, or ciprofloxacin. Anthrax spores have been used as a bioterrorism weapon, and up to 60% of untreated cases of some forms may result in death.