Bacillus anthracis is the etiologic agent of anthrax - a common disease of livestock and, occasionally, of humans - and the only obligate pathogen within the genus Bacillus. B. anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium, with a width of 1.0-1.2 µm and a length of 3-5 µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.