Anthrax spores are formed by anthrax bacteria that occur naturally in soil in most parts of the world. The spores can remain dormant for years until they find their way into a host. Common hosts for anthrax include wild or domestic livestock, such as sheep, cattle, horses and goats.
Although rare in the United States, anthrax is still common throughout the developing world, such as in Iran, Iraq, Turkey, Pakistan and sub-Saharan Africa.
Most human cases of anthrax occur as a result of exposure to infected animals or their meat or hides. In the United States, a few people have developed anthrax while making traditional African drums from the skins of infected animals.
One of the few known instances of nonanimal transmission occurred in the United States in 2001 when 22 people developed anthrax after being exposed to spores sent through the mail. Five of those who were infected died.
More recently, 54 heroin users in Europe contracted anthrax through injecting illegal drugs. Eighteen people died from injectable anthrax. Heroin sold in Europe likely comes from areas where naturally occurring anthrax is more common.