A virus is a small, infectious agent that consists of a core of genetic material (either deoxyribonucleic acid [DNA] or ribonucleic acid [RNA]) surrounded by a shell of protein. Viruses cause disease by infecting a host cell and commandeering the host cell's synthetic capabilities to produce more viruses. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the host. Because viruses invade cells, drug therapies have not yet been designed to kill viruses, although some have been developed to inhibit their growth. The human immune system is the main defense against a viral disease.

Viruses can infect both plants, bacteria, and animals. The tobacco mosaic virus, one of the most studied of all viruses, infects tobacco plants. Bacterial viruses, called bacteriophages, infect a variety of bacteria, such as Escherichia coli, a bacteria commonly found in...