Anthrax is an infection caused by the bacterium Bacillus anthracis. It can occur in four forms: skin, lungs, intestinal, and injection. Symptom onset occurs between one day and more than two months after the infection is contracted. The skin form presents with a small blister with surrounding swelling that often turns into a painless ulcer with a black center. The inhalation form presents with fever, chest pain and shortness of breath. The intestinal form presents with diarrhea (which may contain blood), abdominal pains, nausea and vomiting. The injection form presents with fever and an abscess at the site of drug injection.

According to the Centers for Disease Control and Prevention in the United States, the first clinical descriptions of cutaneous anthrax were given by Maret in 1752 and Fournier in 1769. Before that anthrax had been described only through historical accounts. The Prussian scientist Robert Koch (1843–1910) was the first to identify Bacillus anthracis as the bacterium that causes anthrax.

Anthrax is spread by contact with the bacterium's spores, which often appear in infectious animal products. Contact is by breathing or eating or through an area of broken skin. It does not typically spread directly between people. Risk factors include people who work with animals or animal products, travelers, and military personnel. Diagnosis can be confirmed by finding antibodies or the toxin in the blood or by culture of a sample from the infected site.

Anthrax vaccination is recommended for people at high risk of infection. Immunizing animals against anthrax is recommended in areas where previous infections have occurred. A two-month course of antibiotics such as ciprofloxacin, levofloxacin and doxycycline after exposure can also prevent infection. If infection occurs, treatment is with antibiotics and possibly antitoxin. The type and number of antibiotics used depend on the type of infection. Antitoxin is recommended for those with widespread infection.

A rare disease, human anthrax is most common in Africa and central and southern Asia. It also occurs more regularly in Southern Europe than elsewhere on the continent and is uncommon in Northern Europe and North America. Globally, at least 2,000 cases occur a year, with about two cases a year in the United States. Skin infections represent more than 95% of cases. Without treatment the risk of death from skin anthrax is 23.7%. For intestinal infection the risk of death is 25 to 75%, while respiratory anthrax has a mortality of 50 to 80%, even with treatment. Until the 20th century anthrax infections killed hundreds of thousands of people and animals each year. Anthrax has been developed as a weapon by a number of countries. In herbivorous animals infection occurs when they eat or breathe in the spores while grazing. Animals may become infected by killing and/or eating infected animals.