What is "Alzhiemer's"?

A classic interchange at the doctor:

• Doctor: Now did you have Alzheimer's in your family medical history? You kind of marked on top of it here.
• Michelle: Oh, I did? Let me see.
• Doctor: [Shows Michelle the paper.]
• Michelle: Oh, I was just correcting it because "Alzheimer's" was spelled wrong.
• Doctor: [Chuckle.] Oh it is? I didn't even notice that. [Another chuckle.]
• Michelle: Yeah, I kind of edit everything. Ha ha.