Measles is caused by a virus. Measles is an extremely contagious, vaccine-preventable disease. The measles, mumps and rubella (MMR) vaccine is highly effective at preventing transmission of measles. One dose of MMR vaccine is approximately 93% effective at preventing measles. Two doses are approximately 97% effective.

Measles is spread from person to person through the air by infectious droplets and is considered highly contagious.

Symptoms include fever, runny nose, cough, loss of appetite, conjunctivitis, and a rash. The rash usually lasts 5-6 days and begins at the hairline, moves to the face and upper neck, and proceeds down the body.

It takes an average of 10-12 days from exposure to the first symptom, which is usually fever. The measles rash doesn't usually appear until approximately 14 days after exposure, 2-3 days after the fever begins.

Measles can be a serious disease, with 30% of reported cases experiencing one or more complications. Death from measles occurs in 2 to 3 per 1,000 reported cases in the United States. Complications from measles are more common among very young children (younger than 5 years) and adults (older than 20 years). Some people who become sick with measles also get an ear infection, diarrhea, or a serious lung infection, such as pneumonia. Although severe cases are rare, measles can cause swelling of the brain and even death. Measles can be especially severe in infants and in people who are malnourished or who have weakened immune systems (such as from HIV infection or cancer or from certain drugs or therapies).

Diarrhea and ear infections are common complications of measles. More severe complications may also occur. As many as one out of every 20 children with measles gets pneumonia, the most common cause of death from measles in young children. About one child out of every 1,000 who get measles will develop encephalitis (swelling of the brain) that can lead to convulsions and can leave the child deaf or with intellectual disability. For every 1,000 children who get measles, one or two will die from it. Measles may cause pregnant woman to give birth prematurely or have a low-birth-weight baby.

Before the measles vaccination program started in 1963, an estimated 3 to 4 million people got measles each year in the United States. Of these, approximately 500,000 cases were reported each year to CDC; of these, 400 to 500 died, 48,000 were hospitalized, and 1,000 developed encephalitis (brain swelling) from measles. Since then, widespread use of measles vaccine has led to a greater than 99% reduction in measles cases compared with the pre-vaccine era. However, measles is still common in other countries. Unvaccinated people continue to get measles while abroad and bring the disease into the United States and spread it to others.

Measles is highly contagious and can be transmitted from four days before the rash becomes visible to four days after the rash appears.

Notification of the exposure should be communicated to a healthcare provider and the local health department. If the person has not been vaccinated, measles vaccine (MMR) may prevent disease if given within 72 hours of exposure. Immune globulin (a blood product containing antibodies to the measles virus) may prevent or lessen the severity of measles if given within six days of exposure.