From: Ask the Experts about Measles, Mumps, and Rubella (MMR) Vaccines - CDC experts answer Q&As
(a site sponsored partially by the CDC)
How serious are measles, mumps, and rubella?
Measles can lead to serious complications and death, even with modern medical care. The 1989–1991 measles outbreak in the U.S. resulted in more than 55,000 cases and more than 100 deaths. In the United States, from 1987 to 2000, the most commonly reported complications associated with measles infection were pneumonia (6%), otitis media (7%), and diarrhea (8%). For every 1,000 reported measles cases in the United States, approximately one case of encephalitis and two to three deaths resulted. The risk for death from measles or its complications is greater for infants, young children, and adults than for older children and adolescents.
Mumps most commonly causes fever and parotitis. Up to 25% of persons with mumps have few or no symptoms. Complications of mumps include orchitis (inflammation of the testicle) and oophoritis (inflammation of the ovary). Other complications of mumps include pancreatitis, deafness, aseptic meningitis, and encephalitis.
Rubella is generally a mild illness with low-grade fever, lymphadenopathy, and malaise. Up to 50% of rubella virus infections are subclinical. Complications can include thrombocytopenic purpura and encephalitis. Rubella virus is teratogenic and infection in a pregnant woman, especially during the first trimester can result in miscarriage, stillbirth, and birth defects including cataracts, hearing loss, mental retardation, and congenital heart defects.