Anthrax is a serious zoonotic disease that can affect most mammals and several species of birds, but is particularly important in herbivores. This disease is caused by a spore-forming bacterium, Bacillus anthracis. Anthrax spores are extremely resistant to inactivation by heat or chemicals, and can survive in the environment for decades. Susceptibility to clinical disease varies, with domesticated and wild ruminants most susceptible, horses somewhat less susceptible, and omnivores and carnivores relatively resistant. In endemic regions, anthrax can be a serious problem in unvaccinated ruminants. Although antibiotics may be effective if started early, the course of disease is usually rapid in these animals, and symptomatic infections are often fatal. Epizootics in wildlife are also a concern. In 2004, an outbreak in the Malilangwe Wildlife Reserve in Zimbabwe killed almost all of the approximately 500 kudu in the reserve, as well as large numbers of other wild ruminants.

Human cases usually develop after exposure to infected animals and their tissues. In most countries, human anthrax occurs infrequently and sporadically, mainly as an occupational hazard among veterinarians, agricultural workers and workers who process hides, hair, wool and bone products. In humans, the three forms of anthrax are cutaneous, gastrointestinal and inhalational. Cutaneous anthrax accounts for more than 95% of natural infections, and is rarely fatal if treated with antibiotics. The gastrointestinal form is less common but more serious, and can occur in outbreaks associated with contaminated meat. Inhalational anthrax is the most serious form, and has a very high case fatality rate even when treated. Natural cases of inhalational anthrax are rare; however, anthrax has been used as a weapon by bioterrorists, and weaponized anthrax can form aerosols readily. In 2001, weaponized anthrax was delivered in letters through the United States mail, resulting in 11 cases of inhalational anthrax and 11 cases of cutaneous anthrax. Five people with inhalational anthrax died. Because ruminants are particularly sensitive to anthrax, widespread disease in animals might serve as an early warning of a bioterrorist attack under some circumstances.