> Another (albiet remote) possibility is that a large colony of hyperactive dust mites landed on your neck and are doing what dust mites do best (namely, eating skin cells).

VAGUE SPOILER WARNING!

Interesting that I recently read a short story in _The Year's Best Science Fiction Twenty-First Annual Collection_ about a mysterious flesh-eating disease that turned out to be mutated dust mites that had developed a taste for living skin cells. The protagonist of the story eventually figured out what was going on and cured himself by dousing himself in insecticide.