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Koch's postulates are a set of guidelines developed by Robert Koch, a German scientist and physician, in the late 19th century. These postulates are used to establish the link between a specific microorganism and a particular disease.

There are four main postulates that make up Koch's postulates: 1. The microorganism must be present in every case of the disease but not in healthy individuals. - This implies that every person suffering from the disease must have the suspected microorganism within their body, while healthy individuals must not. 2. The microorganism must be isolated from the diseased individual and grown in pure culture. - This step ensures that the microorganism in question can be studied and analyzed in isolation without the interference of other pathogens or substances. 3. The cultured microorganism should cause the disease when reintroduced into a healthy individual. - To validate the causative relationship, the microorganism must cause the same disease when administered to a healthy individual, proving that it indeed causes the disease in question. 4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original organism. - This final step confirms that the microorganism introduced into the healthy individual is the same one responsible for causing the disease in the original infected person.

Koch's postulates are primarily used to: 1. Establish a causal relationship between a specific microorganism and a particular disease. 2. Provide evidence for the connection between the suspect microorganism and the disease in question. 3. Assist in the identification of the cause of a disease, which is crucial for proper diagnosis, treatment, and prevention.

Although Koch's postulates have been instrumental in identifying the cause of many diseases, they do have certain limitations: 1. Not all microorganisms can be cultured in the laboratory, making it difficult to fulfill the second postulate. 2. Some microorganisms have different strains, which can cause different diseases or varying severity, making it hard to fulfill the third postulate. 3. Some diseases, such as viral infections, have no pure culture, which again makes it challenging to fulfill the second postulate. 4. Some diseases can be caused by multiple microorganisms, which means that not every case of the disease may have the same microorganism present, violating the first postulate. Despite these limitations, Koch's postulates have played a vital role in demonstrating the relationship between specific microorganisms and diseases.