Measles (rubeola) is a highly contagious, infectious disease caused by a single-stranded RNA virus within the Paramyxoviridae family. The disease is worldwide in distribution. The infection primarily occurs in late winter and spring when individuals are in close contact. Cases are more common in developing countries, as the majority of individuals in industrialized nations have been vaccinated. Classically, the disease is more often seen in children. In an unvaccinated population, children younger than 5 years are at highest risk of infection and death. The disease runs a more severe course in malnourished children.

Measles is transmitted via respiratory droplets and can remain viable and transmissible for up to 2 hours in an airspace after an infected person leaves an area. The incubation period after the measles virus enters the upper respiratory mucosa is about 10 days (range 7-21 days), with the rash typically appearing about 14 days after an exposure. Infected individuals are considered contagious from about 4 days prior to the appearance of the rash through about 4 days after its appearance.

A prodrome characterized by fever (up to 40.5°C [105°F]), coryza (nasal congestion), cough, 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 US Centers for Disease Control and Prevention (CDC) reports that approximately 1 in 10 children with measles will develop otitis media, and up to 1 in 20 will develop pneumonia. Encephalitis is a complication in about 1 in 1000 cases.

Subacute sclerosing panencephalitis (SSPE) is a delayed neurodegenerative disorder occurring approximately 10-11 years after acute infection. This complication is characterized by changes in personality, seizures, and coma and eventuates in death.

Although measles was declared eliminated in the United States in 2000, outbreaks 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 was a high number of reported measles cases in the United States in 2014, with 667 cases reported to the CDC, largely from underimmunized communities in Ohio. In 2019, however, an even higher number of confirmed cases was documented: over 1250 cases from 31 states. Since then, the following numbers of cases have been reported annually in the United States: 13 cases in 2020 (8 jurisdictions [jurisdictions include any of the 50 states, New York City, and the District of Columbia]), 49 cases in 2021 (5 jurisdictions), and 121 cases in 2022 (6 jurisdictions). As of February 28, 2023, 3 measles cases have been reported in 2 jurisdictions, including a confirmed case of measles in an individual who attended a large spiritual revival gathering in Kentucky while contagious in mid-February. Refer to the CDC for the latest information on measles cases and outbreaks in the United States.

Related topic: atypical measles