How does one define a virus?
Viruses are extremely small organisms, and their diminutive size was the basis of their original definition. In 1892 D. Ivanovski was studying tobacco mosaic disease. He passed some juice squeezed from an infected tobacco plant through a ceramic filter having small enough pores to filter out bacteria. If the tobacco mosaic disease were caused by a bacterium, he reasoned, this filtering process should effectively sterilize the filtrate. What he found, however, was that the filtered tobacco juice retained the capacity to infect other tobacco plants. The infective agent, being too small to be bacterial, thus became known as a filterable virus. When later experimentation uncovered viruses too large to pass through such filters, other means of definition for such organisms were needed.

In 1933 Wendell Stanley, working at the Rockefeller Institute, was able to crystalize the pure tobacco mosaic virus (TMV). Upon dissolving the crystals and touching the solution to a healthy tobacco plant, infection occurred. This helped to further define viruses, but other significant findings also contributed to forming an accurate definition of these organisms. For example, viruses cannot be grown on a nutrient medium in the laboratory; they can be grown only inside of living cells. Furthermore, viruses do not have the mechanism for self-replication; in particular, they lack an adequate energy reservoir. Because viruses can be grown only inside of living cells, the invention and improvement of tissue culture methods greatly aided the study of these organisms.

It is convenient to classify viruses into four major groups: animal viruses (including viruses affecting humans and other vertebrates), insect viruses, plant viruses, ,and bacterial viruses (bacteriophages, or phages). In a botany course, one would expect to focus attention first on the third group; but investigations involving bacteriophages are especially pertinent and, thus, are addressed immediately following.