Measles is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses. Symptoms include fever, cough, runny nose (coryza), red eyes (conjunctivitis) and a generalized, maculopapular (maculopapular rash), erythematous rash.

Measles is spread through respiration (contact with fluids from an infected (infection) persons nose and mouth, either directly or through aerosol (Particulate) transmission), and is highly contagious—90% of people without immunity (immune system) sharing a house with an infected person will catch it. The infection has an average incubation period of 14 days (range 6-19 days) and infectivity lasts from 2-4 days prior, until 2-5 days following the onset of the rash (i.e. 4-9 days infectivity in total).

An alternative name for measles in English-speaking countries is rubeola, which is sometimes confused with rubella (German measles); the diseases are unrelated. In some other European languages, rubella and rubeola are synonyms, and rubeola is not an alternative name for measles.