Biologists first suspected the existence of viruses near the end of the nineteenth century. At that time, European scientists were seeking to identify the infectious agent responsible for tobacco mosaic disease, a disease that stunts the growth of tobacco plants and makes their leaves blotchy. One experiment used fine-pored porcelain filters to strain bacteria from sap extracted from infected tobacco plants. The pores in these filters were so small that bacteria could not pass through them. However, the infectious agent passed through the filters without difficulty. …
