How do scientists know that a pathogen cause specific disease?

Koch's Postulates are a set of criteria used to determine whether or not a specific microorganism is the causative agent of a particular disease. These criteria were first proposed by the German physician Robert Koch in the late 19th century, and they remain an essential part of medical research today.

The four Koch's Postulates are as follows:

1. The microorganism must be present in every case of the disease. This means that the microorganism must be found in all individuals who have the disease, and it must not be found in healthy individuals.

2. The microorganism must be grown in pure culture. This means that the microorganism must be isolated from the body of an infected individual and grown in a laboratory setting, free from any other microorganisms.

3. The pure culture of the microorganism must cause the disease in healthy animals. This means that when the pure culture of the microorganism is injected into healthy animals, the animals must develop the same symptoms of the disease as the infected individuals.

4. The microorganism must be re-isolated from the diseased animals. This means that the same microorganism that was injected into the healthy animals must be recovered from the animals after they have developed the disease.

If all of these criteria are met, then it is considered to be proven that the microorganism is the causative agent of the disease.

Koch's Postulates have been used to identify the causative agents of many diseases, including tuberculosis, cholera, and anthrax. They remain an important tool for medical research, and they continue to be used to investigate new and emerging infectious diseases.