Measles, also known as rubeola, is a disease caused by a virus, specifically a paramyxovirus of the genus Morbillivirus.

Measles is spread through respiration (contact with fluids from an infected person's nose and mouth, either directly or through aerosol transmission), and is highly contagious—90% of people without immunity sharing a house with an infected person will catch it. Airborne precautions should be taken for all suspected cases of measles.

The incubation period usually lasts for 4-12 days (during which there are no symptoms). Infected people remain contagious from the appearance of the first symptoms until 3-5 days after the rash appears.

Reports of measles go as far back to at least 600 B.C. however, the first scientific description of the disease and its distinction from smallpox is attributed to the Persian physician Ibn Razi (Rhazes) 860-932 who published a book entitled "The Book of Smallpox and Measles" (in Arabic: Kitab fi al-jadari wa-al-hasbah). In roughly the last 150 years, measles has been estimated to have killed about 200 million people worldwide. In 1954, the virus causing the disease was isolated from an 11-year old boy from the US, David Edmonston, and adapted and propagated on chick embryo tissue culture. To date, 21 strains of the measles virus have been identified. Licensed vaccines to prevent the disease became available in 1963.

German measles is an unrelated condition caused by the rubella virus.