Cancer, basically.
Why do you seems to think that makes your past murky? I mean, I don't envy you, it's a horrible disease, but it doesn't make your past murky.
I don't think anyone will consider me unreliable, but for someone going through a stack of applications it's something that sticks out for the wrong reasons.
Plus, an application can be seen by a lot of people and you don't want to start a new job being THAT guy. I don't mind people knowing, but I like to have it on my terms if possible.