Within Celestia's inner circle, it is well-known she is not Immortal, but simply long-lived. It is also known that she has never been sick a day in her life. So when she comes down with a seemingly incurable disease, every one of the doctors who examine her are completely baffled.

Which leaves only one option.

As The Lord of Chaos, Discord has known about a certain Human doctor from Plainsboro, New Jersey for some time, and has been greatly amused by him. What better way to save his friend, while at the same time piss the hell out of both them?