For 1 or 2 weeks after an infection, the virus multiplies without causing any symptoms. This is called the incubation period. After that, symptoms such as cough, fever, runny nose, red eyes, and tearing will appear. Children may also become irritable.
About 2 days after initial symptoms appear, small, red, irregular spots with a whitish or bluish centre called Koplik's spots develop on the inside of the cheeks near the molars. About 2 days after Koplik's spots develop, a rash (large brown or red blotches) develops behind the ears, and perhaps on the forehead and face, and can spread to the trunk, arms, and legs. The rash usually begins to fade within 5 days, often starting at the top (head) before it clears up in the lower parts of the body (legs). Once the rash has completely disappeared, the skin may appear a bit brownish and the top skin layers may peel off soon afterwards.
The rash isn't painful and does not itch. Some people may experience increased sensitivity to light, and their eyes may become red and inflamed. During the peak of the infection, a fever usually develops with a body temperature as high as 40°C (104°F).
Measles is contagious for about 4 days before the rash appears and about 5 days afterwards. It's best for people with measles to stay away from others so they won't get infected.
Measles usually runs a simple course with few complications, but in certain cases, problems can arise. The most common complication is pneumonia, an infection of the lungs. Measles does not cause severe pneumonia itself, but it ties up the immune system and inflames the lungs so that bacteria can easily invade and "super-infect" the lungs. The signs of a bacterial infection include a severe cough that lasts for more than 5 days and yellowish or greenish sputum. If these symptoms develop, a doctor should be consulted right away. A second common bacterial infection that occurs as a result of tissue inflammation caused by the measles virus is a middle ear infection (otitis media).
In about 1 in 1,500 cases, measles can affect the brain, causing encephalitis. This usually happens during the late stages of infection, once the rash has already developed. Unfortunately, there's no cure, but some people may recover on their own without any further problems. Many people, however, are left with permanent problems such as seizures.
In a small number of patients, the virus can take permanent rest in the brain and be awakened years later to cause re-infection and brain damage. This results in a condition called subacute sclerosing panencephalitis (SSPE), which is often fatal. This is very rare, occurring in about 14 out of every 1 million cases of measles.
Measles can also cause complications such as hepatitis (inflammation of the liver) or appendicitis (inflammation of the appendix). Very rare complications include heart and kidney problems.
Pregnant women who catch measles have a greater risk of miscarriage.