I'd just say, "Oh, some medical issues, don't really want to talk about it. But thanks for asking!" And leave it at that. If they keep pressing, just say it's no big deal and you appreciate their concern.

Or you can say "chronic fatigue." That one's pretty ambiguous. I mean, I imagine your experience did take a lot out of you. So it wouldn't be a total lie.

There are certain types of people who are really good at getting you to tell them things you don't necessarily want to talk about, because they come off as really nice and compassionate and understanding, and afterwards you're just like, "Why did I just tell them that?" So just be wary. Some who come off as really nice and compassionate and understanding absolutely can keep a secret, but others cannot. And if your workplace is like high school, then I'm sure there are those who would love to get their hands on a juicy bit of gossip. So keep telling everyone the same thing.

I'm lucky I work on what some of us refer to as The Island of Misfit Toys. I really don't have to worry about hiding much.