This type of thing has never crossed my mind. Maybe I am toooo lax about these things, but I could never see putting something like that on an invitation, unless my child was having issues with his current health status.
I do remember a time though, where one of my friends son had pnemonia, and my child had been over their house playing with her other non-sick sibling. This made me upset. I tried to politely bring the subject up, and she immediately got real defensive and said, "Well the Doctor told me it was not contagious"........but I still did not want my CF child about her sick kid. But then I'm sure he's been around pleanty of other sicker kids at school, that I never even knew aobut, so I guess that is why I don't get hung up on other sick kids.
Colds are colds, maybe my opinion will change or should the day his we have health issues.