Measles is caused by a virus. The infection is spread by contact with droplets from the nose, mouth, or throat of an infected person. The incubation period is 8 to 12 days before symptoms generally appear. Immunity to the disease occurs after vaccination or active infection. Before widespread immunization, measles was so common during childhood that the majority of the population had been infected by age 20. Measles cases dropped over the last several decades to virtually none in the U.S. and Canada because of widespread immunization, but rates have crept up again recently. Some parents are refusing to have their children vaccinated because of 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 the development of autism, however, lower vaccination rates can cause outbreaks of measles, mumps, and rubella -- which can be serious.
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