During harsh conditions, the large, rod-shaped bacteria that cause anthrax develop into spores that can be released into the air and inhaled. Inhaled spores are swept into the trachea, which divides into the bronchus. Because of their small size, the spores are carried into smaller and smaller bronchioles, eventually reaching the tiny alveolar sacs of the lungs.

In the alveolar spaces, immune cells, called macrophages, engulf the spores and attack them with enzymes. Although some spores are destroyed, many others survive the immune attack. The surviving spores travel through the lymphatic system and accumulate inside lymph nodes in the chest.

After a period of time, the spores will germinate and grow into full-fledged anthrax bacteria, which replicate within the lymph nodes. As these bacteria multiply, they produce harmful toxins that cause the lymph nodes to swell and hemorrhage. The toxins also travel through the bloodstream, rapidly producing the dramatic symptoms of anthrax.

Within a few days after the initial exposure, cold-like symptoms including fever, cough, aches, and general malaise develop. Although some people experience a brief recovery, this is followed by a rapid onset of more serious symptoms. During this time, sores develop in the lung tissue where the bacteria first entered the body, and fluid builds up within the chest cavity. This produces bleeding and swelling and restricts breathing.

The toxins also reach the brain and spinal cord, causing bleeding within the thin layer of tissue that encases these structures. The results are severe respiratory problems, hemorrhaging, shock, and often death. However, these dire results can possibly be avoided by taking the anthrax vaccine prior to exposure or by taking antibiotics such as penicillin or cipro shortly thereafter.