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Apparently...
mumps (n.) Look up mumps at Dictionary.com
type of contagious disease, c. 1600, from plural of mump "a grimace" (1590s), originally a verb, "to whine like a beggar" (1580s), from Dutch mompen "to cheat, deceive," originally probably "to mumble, whine," of imitative origin. The infectious disease probably so called in reference to swelling of the salivary glands of the face and/or to painful difficulty swallowing. Mumps also was used from 17c. to mean "a fit of melancholy."
Are mumps always plural?
Can you get only one mump?