Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals. There are effective vaccines against anthrax, and some forms of the disease respond well to antibiotic treatment.

Like many other members of the genus Bacillus, Bacillus anthracis can form dormant endospores (often referred to as "spores" for short, but not to be confused with fungal spores) that are able to survive in harsh conditions for decades or even centuries. Such spores can be found on all continents, even Antarctica. When spores are inhaled, ingested, or come into contact with a skin lesion on a host they may reactivate and multiply rapidly.

Anthrax commonly infects wild and domesticated herbivorous mammals that ingest or inhale the spores while grazing. Ingestion is thought to be the most common route by which herbivores contract anthrax. Carnivores living in the same environment may become infected by consuming infected animals. Diseased animals can spread anthrax to humans, either by direct contact (e.g., inoculation of infected blood to broken skin) or by consumption of a diseased animal's flesh.

Anthrax spores can be produced in vitro and used as a biological weapon. Anthrax does not spread directly from one infected animal or person to another; it is spread by spores. These spores can be transported by clothing or shoes. The body of an animal that had active anthrax at the time of death can also be a source of anthrax spores.

Until the twentieth century, anthrax infections killed hundreds of thousands of animals and people each year in Australia, Asia, Africa, North America, and Europe, particularly in the concentration camps during WWII. French scientist Louis Pasteur developed the first effective vaccine for anthrax in 1881. Thanks to over a century of animal vaccination programs, sterilization of raw animal waste materials and anthrax eradication programs in North America, Australia, New Zealand, Russia, Europe, and parts of Africa and Asia, anthrax infection is now relatively rare in domestic animals, with only a few dozen cases reported every year. Anthrax is especially rare in dogs and cats, as is evidenced by a single reported case in the USA in 2001. Anthrax typically does not cause disease in carnivores and scavengers, even when these animals consume anthrax-infected carcasses. Anthrax outbreaks do occur in some wild animal populations with some regularity. The disease is more common in developing countries without widespread veterinary or human public health programs.

Bacillus anthracis bacterial spores are soil-borne, and, because of their long lifetime, they are still present globally and at animal burial sites of anthrax-killed animals for many decades; spores have been known to have reinfected animals over 70 years after burial sites of anthrax-infected animals were disturbed.

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