Koch's postulates

Koch's postulates are a set of four rules that scientists use to figure out whether a certain disease is caused by a certain type of germ (like a virus or bacteria).

Rule 1: The germ has to be found in every person who has the disease.

Rule 2: The germ has to be taken from a sick person and grown in the lab.

Rule 3: When the germ from a sick person is put into a healthy person, the healthy person has to get the same disease.

Rule 4: The germ has to be taken out of the new person and grown in the lab again.

In other words, Koch's postulates are a way to prove that a certain type of germ is causing a certain type of disease.