Medical Definition of measles
1a: an acute contagious disease that is caused by a paramyxovirus of the genus Morbillivirus (species Measles virus), that commences with catarrhal symptoms, conjunctivitis, cough, and Koplik's spots on the oral mucous membrane, and that is marked by the appearance on the third or fourth day of an eruption of distinct red circular spots which coalesce in a crescentic form, are slightly raised, and after the fourth day of the eruption gradually decline—called also b: any of various eruptive diseases (as German measles)
2: infestation with or disease caused by larval tapeworms in the muscles and tissues; specifically : infestation of cattle and swine with cysticerci of tapeworms that as adults parasitize humans—compare measle
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