The study of viruses, or virology as it is now called, had its origin in 1892 when a Russian botanist, Iwanawsky, showed that sap from a tobacco plant with an infectious disease was still highly infectious after passage through a filter capable of retaining bacterial cells. From such humble beginnings the study of these 'filter-passing agents', or viruses, has developed into a separate science which rivals, if it does not excel, in importance the whole of bacteriology. The importance of viruses lies not only in the diseases they cause in every type of living organism, but also because of...