Koch's Postulates

Koch's postulates, also known as Henle-Koch postulates, were published by Robert Koch in various forms between 1878 and 1884 to set forth a method of demonstrating that a bacillus causes a particular disease. These postulates follow the process that Koch went through in demonstrating that anthrax and tuberculosis bacilli cause disease. Koch's postulates state that, to establish that an organism causes disease,

• the organism must be present in all cases of the disease;
• the organism must be grown in pure culture outside a diseased animal;
• when inoculated with the organism, healthy test animals must develop the same symptoms as were present in the original cases; and
• the organism must be present in the experimentally infected animals.

Koch believed that satisfying these postulates provided definitive proof that the organism was a necessary and sufficient cause of disease. If fulfilled, these postulates provide powerful evidence that an agent causes disease; however, all these conditions need not be fulfilled to establish causation. Koch noted in his investigations that healthy animals sometimes would not develop disease after being inoculated with the pathogen, leaving the third postulate unfulfilled. Such asymptomatic infections occur in many diseases with well-established causes, such as cholera and influenza. The second postulate, that the organism must be grown in a pure culture outside the diseased animal, cannot be fulfilled for viruses since they are intracellular parasites.

Even when not fulfilled in the strictest sense, Koch's postulates still provide guidelines for establishing disease causation. In modern practice, the first and third postulates are more accurately stated as follows:

• The agent must be significantly more common in individuals with the disease than those without.
• Individuals exposed to the agent must be significantly more likely to develop the disease than those who are not.

There is some debate about the extent to which Koch was influenced by the work of Jacob Henle. Those who feel that Koch was heavily influenced by Henle's work on disease causation refer to the postulates as the Henle-Koch postulates or even as the Henle postulates. While the extent to which Koch was influenced by previous investigators is open to some debate, it is clear that the postulates were significant in Koch's groundbreaking work showing the role of the anthrax and tuberculosis bacilli in the causation of disease. The postulates therefore generally bear only his name.

Modern epidemiological research often focuses on diseases, such as diabetes and cancer, that are not necessarily caused by microorganisms. For these diseases other methods for establishing causation, such as Hill's Considerations for Causal Inference, are used instead of Koch's postulates. An expansion of Koch's postulates was proposed by Alfred S. Evans, who attempted to unify criteria of causation used in the investigation of chronic and acute diseases.