Dude. Joking about Alzheimer's isn't funny.
There are dozens of cancer jokes, leukemia jokes, down-syndrome jokes, Alzheimer's jokes... It's called "black humour", and it's distasteful as hell, but it does tend to be funny. Occasionally. My grandmother died of cancer. My aunt died of leukemia. My own mother is a cancer survivor. None of them were/are offended by mere lack of taste. I even met this Jewish dude (a Trekkie BTW, and an Israeli national) who didn't even flinch when some dickhead told one of those godawful holocaust jokes...
But ironically, I wasn't trying to be funny. It was just another way of saying "you're full of shit". Like when you call someone "retarded", for saying something stupid. You don't actually mean to insult mentally handicapped people with that.