Measles is a highly contagious infection of the respiratory system caused by the measles virus, which is found worldwide. The virus multiplies in cells of the respiratory tract in infected persons and is released into fluids of the nose, throat and mouth.
Although measles continues to be a problem in developing countries, U.S. measles cases have been decreasing since 1993. The decrease can be attributed to: 1) increased immunization coverage, 2) the use of a two-dose vaccine strategy for children, and 3) programs to eliminate measles in Mexico and Central and South America. Measles can be prevented by immunization. About 95% of vaccinated persons are protected with one dose, and practically everyone is protected with two doses.
It is clear that there are risks associated with the various measles vaccines and the debate over whether they should be given to children rages on.
People get measles when they breathe in tiny droplets that contain the virus. The virus attaches to the lining of the airways, multiplies, and causes disease. People can also be infected by direct contact with fluid from the nose or mouth of an infected person. An uninfected person can get measles simply by breathing the air in a room where an infected person has been. The virus can live in the air for 2 hours after an infected person leaves a room. Infected persons are usually infectious to others even before symptoms appear.
Persons who have not been immunized against measles are the main risk group. Infants are generally protected from measles for 6-8 months after birth, due to immunity that was passed on by their mothers. Older children usually receive measles immunization according to state and school health regulations in the MMR vaccine. Outbreaks of measles are currently occurring most often on college campuses, among young persons who have not been adequately immunized against measles or whose immunity has decreased since childhood.