There are a number of different viruses that challenge the human immune system and that may produce disease in humans. In common, viruses are small, infectious agents that consist of a core of genetic material—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—surrounded by a shell of protein. Although precise mechanisms vary, 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...