Naa... I don't think that's wrong. I mean, don't just dump him straight away as soon as you find out he's not ill, but soon after is fine. If you don't want to be with him, there's no point.
And even if he is ill, that doesn't mean you have to stay with him forever. I know it's a different situation, as in this case it wasn't life threatening, but a friend of mine had a stroke last year (aged only 17, which is really weird). His girlfriend (another friend) stayed with him for a while, because he really needed her support. He's still quite ill now, and has changed a lot, but he's not that bad. She broke up with him several months ago, but she stuck with him through the worst.
You can't stick with someone forever just because they're ill. But it would be harsh to break up with them at a really bad time...