Measles is caused by the measles virus, a single-stranded, negative-sense, enveloped RNA virus of the genus Morbillivirus within the familyParamyxoviridae. The virus was first isolated in 1954 by Nobel Laureate John F. Enders and Thomas Peebles, who were careful to point out that the isolations were made from patients who had Koplik's spots. Humans are the only natural hosts of the virus, and no other animal reservoirs are known to exist. This highly contagious virus is spread by coughing and sneezing via close personal contact or direct contact with secretions. Risk factors for measles virus infection include immunodeficiency caused by HIV or AIDS, immunosuppression following receipt of an organ or a stem cell transplant, alkylating agents, or corticosteroid therapy, regardless of immunization status; travel to areas where measles is endemic or contact with travelers to endemic areas; and the loss of passive, inherited antibodies before the age of routine immunization.