Anthrax is caused by the bacterium Bacillus anthracis. While anthrax commonly affects hoofed animals such as sheep and goats, humans may acquire this disease as well. Historically, the main risk factor for acquiring inhalation anthrax is breathing in "aerosolized" anthrax spores from industrial processes such as tanning hides and processing wool.

Anthrax is a potential agent for use as a biological weapon or bio-terrorism. While at least 17 nations are believed to have a biological weapons program, it is unknown how many nations or groups are working with anthrax. Most bio-terrorism experts have concluded that it is technologically difficult to use anthrax effectively as a weapon on a large scale.

Inhalation anthrax develops when anthrax spores enter the lungs. A person may have spores in the nasal passages (which indicates exposure), but that doesn't mean they will get the disease. In fact, antibiotic therapy following known or suspected exposure can help prevent the disease.

In order for a person to develop the actual disease, the spores must germinate -- a process which may take several days, or even up to 60 days to occur. The spores move to the lymph nodes and once they germinate, lead to the release of several toxic substances (toxins). This results in hemorrhage, swelling, and tissue death.

The main form of inhalational anthrax includes hemorrhagic infection of the lymph nodes in the chest (hemorrhagic mediastinitis). Up to half of affected individuals may also have a hemorrhagic meningitis.

There are usually two stages of inhalation anthrax -- the first stage can last from hours to a few days and is similar to a flu-like illness with fever, headache, cough, shortness of breath, and chest pain.

The second stage often develops suddenly and is notable for shortness of breath, fever, and shock. This second stage is fatal in up to 90% of individuals because of the build-up of toxins.