Illustration depicting anthrax bacteria and endospores (small yellow spheres) in the blood stream. The bacteria Bacillus anthracis produces highly toxic endospores (spores) which can exist in the environment for years or even centuries. The spores may become airborne and inhaled by mammals, including humans. The spores become active and travel through the airways to the alveoli in the lungs where they enter the bloodstream. In the alveoli and the blood they are attacked by lymphocytes (center left). Although many are destroyed, the remaining spores continue to reproduce, eventually killing the lymphocyte (lower right). They go on to infect the lymphatic system and the rest of the body, including the brain. If left untreated the infection can become fatal.