Do you tell your employers about your illnesses?

I'm just curious, because for certain things I kind of feel like I have to tell them but for others I just keep it to myself. Like with colds and calling in sick, it's pretty self evident just by talking to me. But for the more serious stuff, I tend to try to ignore it at work unless it interferes with my job somehow (though technically all illnesses interfere with jobs in one way or another). If it won't get others sick, it's no one else's business. But also if it means taking a lot of time off, maybe it has to be theirs?