 For example, Koch was well aware that in the case of cholera, the causal agent, "Vibrio cholerae", could be found in both sick and healthy people, invalidating his first postulate.Furthermore, viral diseases were not yet discovered when Koch formulated his postulates, and there are many viruses that do not cause illness in all infected individuals, a requirement of the first postulate.Additionally, it was known through experimentation that "Helicobacter pylori" caused mild inflammation of the gastric lining when ingested.