How do you ask someone what kind of medical condition they have without sounding like an a$$hole?

You know how there are physical abnormalities you can see but cannot place? Well, I'm a very curious person, and the more I know about a person's condition the closer and more comfortable I feel around them. I do realize that this is a very sensitive subject (hence me risking already being considered an a$$ for even questioning), and so I want a serious answer, if possible. Thanks