The early studies on infectious diseases in the late 19th century quickly demonstrated that microscopic organisms were the cause of many illnesses, and a number of bacteria were isolated by filtering the lysate of diseased cells and examining them by cultivation and microscopy. However, several diseases failed to reveal an agent that could be grown or seen with the microscopes of the day, and yet diseased cell lysates could faithfully transmit the symptoms to new hosts. Many of these first steps into the discovery of viruses (the word ‘virus’ comes from the Latin for ‘poison’) were done using plant viruses and our modern understanding of virus structure, replication, assembly, and even the 320nature of virus genetics was derived from studies of the tobacco mosaic virus (TMV). The virus, which causes mosaic patterning of the leaves of the tobacco plant (see Figure 2), was first described in 1892. TMV was the first virus to be observed under the electron microscope, the first to demonstrate the intrinsic infectivity of a naked viral genome, and the first virus to be assembled in vitro from purified preparations of viral RNA and coat protein molecules. Studies with turnip yellow mosaic virus and tomato bushy stunt virus revealed the morphological details of icosahedral viruses. Ironically, in more recent times the molecular study of plant viruses has trailed behind that of animal viruses, as plant cells are more difficult to culture than animal cells.