The majority of viral infections acquired by nonimmunosuppressed individuals are asymptomatic or result in mild clinical manifestations. These individuals are able to mount effective immune responses that either clear the virus from the body or, for viruses that establish latency, maintain the virus in a dormant state. For individuals who are immunocompromised, because of a genetic immunodeficiency or immunosuppressive drug therapy, viral infections may result in life-threatening disease. Such patients present with infections that are severe, persistent, recurrent, or refractory to therapy and are usually caused by viruses of low virulence prevalent in the community.