Measles is a highly contagious disease caused by the measles virus. Infected people have the measles virus in the mucus of their nose and throat. When they sneeze or cough, moisture droplets spray into the air. The virus in these droplets can remain active on surfaces for up to two hours. The virus is spread by coming in contact with these infected droplets.

Following exposure to the measles virus, there is usually an incubation period lasting 10 to 12 days, during which there are no signs of the disease. During this time, the virus begins to multiply and infect the cells of the respiratory tract, eyes, and lymph nodes—increasing the levels of the virus in the blood stream.

The first stage of the disease begins with a runny nose, cough, and a slight fever. As the infection progresses, the person's eyes become red and sensitive to light.

The second stage of measles is marked by a high temperature—sometimes as high as 103° F to 105° F—and the characteristic red blotchy rash. The rash usually starts on the face and then spreads to the chest, back, and arms and legs, including the palms of the hands and soles of the feet. After about 5 days, the rash fades in the same order as it appeared. Tiny white spots, called Koplik’s spots, can also appear in the mouth.

A person with measles can be contagious from up to 4 days before and after the rash appears.

An effective MMR vaccine for measles is usually given in combination with vaccines for mumps and the less severe German measles, or rubella. This vaccine contains weakened or killed forms of the virus, which stimulates the body's immune system to recognize the virus as foreign. Therefore, the immune system can more easily identify and kill any of these viruses that it encounters in the future.