Viruses are tiny particles that consist mainly of protein and nucleic acid such as DNA or RNA. Because they contain genetic material, they are able to reproduce, but lacking cell structures and the ability to metabolize food, they can reproduce only inside other living host cells. Viruses, far smaller than the smallest bacteria, can be crystallized, much like a mineral forms a crystalline structure. Thus, viruses are not considered alive. But because they undergo genetic recombination and mutation, viruses are very adaptable and are found throughout the biosphere, from the viruses that cause AIDS, chicken pox, and measles in humans to the viruses that live within plants and cause tobacco leaves to wilt and tulips to turn bright colors.

The existence of viruses was only hypothesized a little over a hundred years ago. In the late nineteenth century, Louis Pasteur, the notable French chemist and microbiologist, suggested that...