[Free Registration/Login may be required to access all resource tools.] Today most people realize that microorganisms, such as bacteria or viruses, are the cause of disease. This concept is known as the germ theory of disease, one of the few scientific theories in the field of the life sciences. Although it seems obvious now, people did not always understand the cause of disease. How does a theory such as this become established? Learn more about scientific theories in this learning module produced by CK-12.