the scientific study of viruses or of the diseases caused by them. Branch of microbiology that deals with the study of viruses. Viruses were not closely examined until 1892, when a Russian bacteriologist observed that the agent of tobacco mosaic disease could pass through a filter that did not permit the passage of bacteria. Modern virology began in the early 20th century, when the existence of bacteriophages was discovered. Direct visualization of viruses became possible after the electron microscope was introduced ( 1940)