#9. History of the Discovery of Viruses

While Koch’s postulates worked well for delineating diseases caused by bacteria and other such organisms, there were diseases that remained mysterious. The one most seriously studied was the tobacco mosaic disease, a disease that causes splotching and wilting of the leaves of tobacco plants. Injection of sap from a diseased plant into healthy plants clearly showed transfer of an infections agent, but the agent could not be seen under light microscopes, much less cultured and identified. It was 1931 before this was accomplished by means of the newly invented electron microscope. Since then, especially in the latter half of the 1900s over 6000 species of viruses have been identified.  

See:  https://www.utmb.edu/virusimages/the-virus-images.

This history of this discovery is described in more detail in the following videos: