The concept of an infectious agent emerged in the mid-19th century and was defined by a German physician, Robert Koch (1843-1910), using Bacillus anthracis in mice. He demonstrated the isolation of this single microorganism from Koch's postulate is the standard for the definition of an infectious agent. In his cases of anthrax, the reproduction of the disease by inoculation of the organism into experimental animals and the re-isolation of the same organism from these experimental cases.