On June 20 in 1920, microbiologist Dmitry I. Ivanovsky, who discovered viruses, dies at 55 in Rostov-on-Don, Russia. He was commissioned in 1890 to study mysterious diseases that were killing tobacco crops in the Crimea. He determined that some agent in sap could transfer the disease from plant to plant, and then (through detailed filtering and microscope work) he concluded that some invisible parasite, much smaller than any known bacterium, was the culprit. He assumed, and reported, that he was working with just a supersmall bacterium, but he had actually discovered a whole new form of life: the virus.
—from The Illustrated Almanac of Science, Technology, and Invention