Measles virus (MeV) is a highly infectious respiratory virus that causes a systemic rash disease associated with substantial morbidity and mortality. Infection results in a viremia followed by persistence of viral RNA in blood and lymphoid tissue for many months, immune suppression and increased susceptibility to other infections. An efficacious and safe live attenuated virus vaccine is in widespread use. Because of the need for high population immunity to prevent endemic transmission, 2 doses are required. Delivery of the second dose in resource poor parts of the world has relied on mass campaigns that have been difficult to sustain and measles has returned in many of these countries. Because of philosophical objections to vaccination, outbreaks of measles have also been increasing in industrialized nations that once had eliminated endemic transmission.