There's a spot on my scalp that's approximately the diameter of a baseball that's itched almost non stop for about a year and a half.
For the first few months, it was hard to resist scratching at it hundreds of times per day. As time has gone on, it's gotten easier to resist the urge to scratch. I can even go hours without even thinking about it anymore.

When I put any pressure on it or when I wash my hair, however, the itchiness amplifies so much that I scratch at it furiously like a chipmunk.
My wife has looked at the area and sees nothing unusual except for a little temporary redness that comes after I've attacked it with my claws.
I do feel tiny bumps here and there, but she says she can't see them.

What is it?

Cancer?
AIDS?
Sickle cell anemia?
Maybe tooth decay?
Kennel cough?