Anthrax is an acute infectious disease caused by the spore-forming bacterium B. anthracis. It can infect humans but is most common in other warm-blooded animals such as herbivores. An intentional release of B. anthracis spores in October 2001 resulted in the death of five persons from inhalation anthrax. The most recent reported case of inhalation anthrax in the United States before October 2001 was in 1976. Since 2001, several cases of anthrax have resulted from natural occurrences-not bioterrorism.