One mump?

We call the disease "mumps." Words that end in "s" are often assumed to be plural. After all, we get "the shingles," and we know that that refers to a multitude of bumps. Would not the same logic apply to mumps? However, even if you only get the disease on one side of your face, it is still "mumps."

I checked in the dictionary, and it did not list "mump," only "mumps." However, Skeat's Etymological Dictionary does list "mump," meaning "to mumble, sulk, beg."