Koch's postulates are a series of conditions that must be met for a microorganism to be considered the cause of a disease. German microbiologist Robert Koch (1843–1910) proposed the postulates in 1890.

Koch originally proposed the postulates in reference to bacterial diseases. However, with some qualifications, the postulates can be applied to diseases caused by viruses and other infectious agents as well.

According to the original postulates, there are four conditions that must be met for an organism to be the cause of a disease. Firstly, the organism must be present in every case of the disease. If not, the organism is a secondary cause of the infection, or is coincidentally present while having no active role in the infection. Secondly, the organism must be able to be isolated from the host and grown in the artificial and controlled conditions of the laboratory. Being able to obtain the microbe in a pure form is...