The Natural History of Severe Viral Infections and Characterization of Immune Defects in Patients Without Known Immunocompromise

Background: * Infections caused by viruses are common causes of illnesses: the common cold, many ear infections, sore throats, chicken pox, and the flu are caused by different viruses. Usually, these illnesses last only few days or, at most, a few weeks. Some virus infections like influenza are cleared from the body, and others such as the chicken pox virus remain in the body in an inactive state. However, some people may become quite ill when they are infected with a particular virus, possibly because part of their immune system does not respond properly to fight the virus. * Researchers have discovered some reasons why a person may not be able to clear an infection caused by a virus. Some persons have changes in the genes that involve the immune system that result in the inability to properly control infection with a particular virus. Identifying changes in genes that involve the immune system should help scientists better understand how the immune system works to protect people from infection and may help develop new therapies. Objectives: * To study possible immune defects that may be linked to a particular severe viral infection. * To determine if identified immune defects are genetic in origin. Eligibility: * Individuals of any age who have or have had a diagnosis of a virus infection that physicians consider to be unusually severe, prolonged, or difficult to treat. * Relatives of the participants with a severe viral infection may also participate in the study. We will use their blood and/or skin specimens to try to determine if identified immune defects are hereditary. Design: * Prior to the study, the participant's doctor will give researchers the details of the infection, along with medical records for review. Eligible participants will be invited to the NIH Clinical Center for a full evaluation as an outpatient or inpatient. * At the Clinical Center, participants will be treated with the best available therapy for the particular viral infection, and researchers will monitor how the infection responds to the treatment. * Researchers will take intermittent blood samples and conduct other tests (such as skin biopsies) to evaluate the immune system. - During and after the illness, researchers will conduct follow-up visits to determine the course of infection and response to therapy.