Measles
Middle English maseles, probably from Middle Dutch masel ‘pustule’ (compare with modern Dutch mazelen ‘measles’). The spelling change was due to association with Middle English mesel ‘leprous, leprosy’.
wiktionary
Plural of Middle English mesel, masel, probably from Middle Dutch masel(“blemish”) due to confusion with measle. Cognate with Middle Low German masele, māsel.
See measle
etymonline
measles (n.)
infectious disease causing eruptions of rose-colored papulae, early 14c., plural of Middle English masel "little spot,"which isperhaps from Middle Dutch masel "blemish" (in plural "measles") or Middle Low German masele, both from Proto-Germanic *mas- "spot, blemish" (source also of 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. "The phonetic development is irregular" [OED] and the form might have been influenced by Middle English mēsel "leprous; a leper; leprosy" (late 13c., obsolete from mid-16c.), which is from Old French mesel and directly from Medieval Latin misellus "a wretch," noun use of an adjective meaning "wretched," a diminutive of Latin miser "unhappy, wretched, pitiable, in distress."
