measles (n.) Look up measles at Dictionary.com
infectious disease, early 14c., plural of Middle English masel, perhaps from Middle Dutch masel "blemish" (in plural "measles") or Middle Low German masele, from Proto-Germanic *mas- "spot, blemish" (cognates: Old High German masla "blood-blister," German Masern "measles").

There might have been an Old English cognate, but if so it has not been recorded. Form probably influenced by Middle English mesel "leprous" (late 13c.).