If you know then you are obliged to tell...it's simple as that. You have no way of knowing the health of other people's children or whose kid is immuno-compromised, allergic, etc. With that said, why would you want to be the contributing factor to what could be a serious situation for some, if indeed you're kids are sick. People can say what they want that you can't track down where germs come from. Not true. It's easier than you want to believe. Pretty much everytime my kids got sick, I knew exactly where they got it and from whom, regardless of whether parents told me or not if it turned out the carrier was a child/friend.
I flipped my lid a year ago when I found out a friend "forgot" to tell me she was purposely exposing her kids (just days prior -that previous weekend to be exact) to another family that was sick for over a month with chicken pox! No mention at all until 4 hours AFTER she returned to pick up her own children. I guess because they didn't have visible symptoms she assumed they were asymptomatic!!??? What the hey?! In case you're wondering, she's anti-vaccine and was trying to boost their immunity the "old fashioned way." (Her words not mine) ;p
My kids are vaccinated, but my husband and I never had them. It can be serious to deadly for adults or those who are pregnant. I thought this friend knew better about common courtesy concerning germs. Apparently not...or worse yet, she was so desperate for a day away from the kids she didn't care!! Needless to say, we don't go out of our way to associate with these folks much anymore. I guess I have trust issues! LOL.
Lucky for her, we didn't get sick, but if we had....ooooh.
I say yes, there are some illnesses you just don't know until it's too late and that can't be helped. But I know too many folks who bring their sniveling, post-nasal drip kids around anyway. I think it's really rotten. People should have the option to bow out and be preventative if they want to. No parent should KNOWINGLY force germs on another family. It's just the right thing to do.