Nobody knows where the dan in dandruff comes from, but the second syllable probably derives from the Old English hreofla meaning leper. Perhaps there was a particular flaky chap called Dan.
I hate to tell you this, but there is a thing called leper juice. It's the stuff in the lesions. The things you find in the OED. I'm off to start a soft drinks company.
Incidentally, the World Health Organisation think leprosy's got a bad reputation and prefer to call it Hansen's Disease. This must be a great comfort to those dying of leprosy, but not to people called Hansen.
Not a speck in sight.