Koch's postulates

Robert Koch, here seen posing for a post-postulation photography.

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Koch's postulates are a set of "rules" formulated by Robert Koch and Friedrich Loeffler in 1884 that establish the relationship between a pathogen and a disease. These were developed early in the history of the germ theory of disease.

To fulfill the original postulates:

1. The microorganism must be found in all organisms suffering from the disease (the original postulates also said "but not in healthy organisms." Since latency is present in many diseases, this phrase is largely abandoned).
2. The microorganism must be isolated from a diseased organism.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Koch’s postulates today

Today, Koch’s postulates are not considered as essential to virology as it once was. Koch himself discovered cases of cholera in healthy individuals, making the claim that viruses are only in sick individuals irrelevant.

Modern molecular biology has changed this schema somewhat, and has also allowed for the development of the so-called "molecular Koch's postulates".

These postulates, in their original form, are often used by HIV denialists to try to show that HIV does not cause AIDS, and were used to some extent by the tobacco industry to deny that smoking causes lung cancer (obviously inappropriate, since there's no microorganism involved). In the case of HIV denialism, HIV does in fact meet the postulates as long as they are not applied in a ridiculously stringent manner. It's further notable that Koch is credited as the father of bacteriology; viruses were unknown in his time.

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