During the second half of the 19th century, bacteria were recognized as the causative agents of many infectious diseases of plants and animals, including humans. The assumption came to be that bacteria or other microorganisms would be found to be responsible for all transmissible diseases. The main technique used to separate such infectious agents from extracts or exudates was that of filtration. It thus came as a great surprise or shock to Ivanowski, working on the tobacco mosaic disease in the 1890s in the Crimea (see 1903), that his filtrates of infected plant extracts retained their infectivity. The same observations were made by Beijerinck (1898) in Holland. But only Beijerinck believed the results of his own experiments, and concluded that he had discovered a new type of infectious agent, “Contagium vivum fluidum.” This term was abandoned when it became apparent that the contagium was neither alive nor fluid. But the concept of the existence of filtrable infectivity, soon called filtrable virus, became quickly accepted, since similar results were obtained by Loeffler and Frosch (1898) working in Germany on the foot-and-mouth disease of cattle, and later by others working on other diseases. This new type of infectious agent, soon termed simply virus, was characterized as being too small to be retained by bacterial filtration methods, too small to be seen using the microscope, and not cultivatable in vitro by any of the methods used by bacteriologists to “grow” microorganisms. Although small, compared to microorganisms, they were nevertheless soon found to be large, compared to even the largest known chemical molecules. This recognition was put on a quantitative basis by the use of filters of varying porosity (Elford, 1932). It was particularly the development of the ultracentrifuge and of the electron microscope that confirmed this fact, and gave us definitive information about the size and shape of viruses.