I had the opposite problem a couple of weeks ago. My DS threw up majestically in the night witha tummy bug and DD was due at a sleepover the following night. I withdrew her from the sleepover in case she either had the bug or was carrying it as she would have been sleeping over with 9 other children 3 days before Easter.
Can you imagine the destruction that one would have caused?!
The reason I did it was that a similar thing happened last term. A group of boys had a sleepover for a friends birthday and they all went down with a horrendous bug in the week that followed, which must have come from one of them carrying it unknowingly. Each child then gave it to their siblings and parents, the end result being that most of the school went down with it within 2 weeks.
Anyway, despite initial tears, DD was fine and didn't go down with the bug herself, and had a lovely day with her friend the following week to make up for the disappointment. BUT I would not have forgiven myself if she had got it and passed it on to all those other children.