Measles (rubeola) is caused by a single-stranded RNA virus of the Paramyxoviridae family. The disease is worldwide in distribution, with sustained outbreaks being reported in many countries. The infection primarily occurs in late winter and spring. Cases are more common in developing countries, as the majority of individuals in industrialized nations have been vaccinated. Classically, the disease is seen more often in children. In an unvaccinated population, children aged younger than 5 years are at highest risk of infection and death.
Although measles was declared eliminated in the United States in 2000, outbreaks resulting from imported cases continue to occur. Most cases are associated with importation of measles by unvaccinated international travelers resulting in local outbreaks in communities with clusters of unvaccinated individuals. There were a high number of reported measles cases in the United States in 2011, with 220 cases reported to the CDC. In 2013, 189 cases were reported. In 2014, a record number of 644 cases from 27 states were reported to the CDC. From January 1 to January 30, 2015, 102 cases from 14 states have been reported, most of them part of a large, ongoing multi-state outbreak linked to an amusement park in California.
Measles is transmitted via respiratory droplets, and it is highly infectious. The incubation period after the measles virus enters the upper respiratory tract and nasal passages is about 10 days (range 7-21 days), with the rash typically appearing about 14 days after a person is exposed. Infected individuals are considered contagious from 4 days before to 4 days after the rash appears.
A prodrome characterized by coryza (nasal congestion), cough, fever (up to 105°F [40.5°C]), and conjunctivitis occurs for about 3-4 days followed by the onset of the rash (sometimes immunocompromised individuals do not develop the rash). The coryza, a “barking” cough, and conjunctivitis will increase in severity until the rash reaches its peak. The CDC reports that approximately 1 of 10 children with measles will also have an ear infection, and up to 1 of 20 will develop a pneumonia. Encephalitis is a complication in about 1 of 1000 cases.