Merriam-Webster Collegiate Dictionary - measles

 noun plural but singular or plural in construction  Etymology: Middle English meseles, plural of mesel ~, spot characteristic of ~; akin to Middle Dutch masel spot characteristic of ~  Date: 14th century  1.  a. an acute contagious disease that is caused by a morbillivirus (species Measles virus) and is marked especially by an eruption of distinct red circular spots  b. any of various eruptive diseases (as German ~)  2. Middle English mesel infested with tapeworms, literally, leprous, from Anglo-French, from Medieval Latin misellus leper, from Latin, wretch, from misellus, diminutive of miser miserable infestation with or disease caused by larval tapeworms in the muscles and tissues