This paper reviews the technical developments in microbiology that led to the discovery of new infectious agents and the effect of these discoveries on establishing proof of causation. In bacteriology, these advances included the light microscope, bacterial stains, bacterial cultures, and the methods used to isolate clones. In virology, they involved the use of filters to separate viruses from bacteria, the electron microscope, the use of laboratory animals, embryonated eggs, tissue cultures to identify or grow the agent, and the recent development of molecular techniques to detect the presence of antigen in tissues. In immunology, they were based on the discovery of antibodies and of the immune response.