A large, gram-positive, rod (bacillus), Bacillus anthracis is the causative agent of anthrax (Greek for “coal”), named for the black lesions of cutaneous anthrax. In 1850, Rayer and Davaine discovered the rods in the blood of anthrax-infected sheep, setting the stage for Koch to link the disease to the bacterium in 1876, after he performed a series of experiments that fulfilled what came to be known as Koch’s postulates. This was among the first times a microorganism was conclusively linked with a specific disease.