Measles (Rubeola) Virus is a community–related acute disease commonly known as red measles or seven-day measles. Measles is one of the infectious diseases of childhood causing a skin rash. Serious complications, such as pneumonia, croup or encephalitis, can occur. It is highly contagious and associated with high morbidity. Measles transmission in preschool-aged children remains a major impediment to the elimination of measles in the United States. Inner city outbreaks commonly occur among preschoolers in low socioeconomic groups in which most affected persons are unvaccinated. There is an increased severity of measles in immunocompromised children with giant cell pneumonia, which is the principal cause of the high mortality from measles in these patients. Today, measles is still one of the major causes of death of young children in many developed countries. Measles Virus not only causes acute disease, in rare instances it can persist and lead to the development of a progressive encephalitis named "sub acute sclerosing panencephalitis" (SSPE). SSPE occurs worldwide but the incidence of this disease has decreased dramatically in the United States following the nationwide measles immunization program.