Measles is a very contagious (easily spread) illness caused by a virus.

The infection is spread by contact with droplets from the nose, mouth, or throat of an infected person. Sneezing and coughing can put contaminated droplets into the air.

Those who have had an active measles infection or who have been vaccinated against the measles have immunity to the disease. Before widespread vaccination, measles was so common during childhood that most people became sick with the disease by age 20. The number of measles cases dropped over the last several decades to almost none in the U.S. and Canada. However, rates have started to rise again recently.

Some parents do not let their children get vaccinated because of unfounded fears that the MMR vaccine, which protects against measles, mumps, and rubella, can cause autism. Large studies of thousands of children have found no connection between this vaccine and autism. Not vaccinating children can lead to outbreaks of a measles, mumps, and rubella -- all of which are potentially serious diseases of childhood.