Etiology and Epidemiology

Measles is caused by a virus, which is in the genus Morbillivirus of the family Paramyxoviridae. Although the virus does not survive drying on a surface, it can survive drying in microdroplets in the air.

Measles is one of the most highly communicable diseases, transmitted by contact of susceptible individuals with the nose and throat secretions of infected persons, primarily by droplet spread. Infection also occurs by direct contact, and by indirect contact through freshly soiled articles and airborne transmission. There is no reservoir for measles other than human beings, which means that a continuous chain of susceptible contacts is necessary to sustain transmission. The period of communicability is from slightly before the beginning of the prodromal phase of the disease to 4 days after the start of the rash. There is no carrier state. Measles has an incubation period from time of exposure to onset of fever of about 10 days with a range from 8 to 13 days. The incubation period from time of exposure to rash onset is about 14 days.

In populated areas with no or low vaccination cov erage, measles is primarily an endemic disease of children, with epidemics occurring every 2 to 5 years. In such areas, the greatest incidence is in children under 2 years of age. Epidemic measles has a winter-spring seasonality in temperate climates and a less marked hot-dry seasonality in equatorial regions. This seasonality may be primarily the result of the indirect effect of climate on socioeconomic conditions and population movements. In more remote isolated populations, measles is not endemic and disease is dependent upon introduction of the virus from the outside, at which time an epidemic may occur, affecting all age groups born since the last epidemic. There is no evidence for a gender difference with respect to incidence or severity of measles, or a racial difference with respect to incidence. Differences in severity among certain populations are most likely the result of nutritional and environmental factors.

Measles mortality is highest in the very young and the very old. In malnourished children in the developing world, the case fatality rate may be as high as 5 to 10 percent or more. Some studies have indicated that multiple cases within a family group may lead to higher mortality rates.

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