You don't want to be one of those parents who is all freaky about your kid getting sick off of another kid. You don't WANT to be that mom who is like, "Oh, your kid coughed. We have to leave the playdate now."
So you write Zen things on Facebook like, "Oh, it's okay that your kid puked last night. We are still coming over for the playdate today." and then after the playdate, when the kid puked AGAIN, you write stuff like, "Oh, I like to just pray and do positive thinking and it will all turn out okay."
And usually it's fine. Your kids have a nice, built-up immunity.
Then they all 5 get sick at once and are puking and you're cursing yourself and your damned positive thinking and Pollyanna sick-itude because plans are cancelled and puke bowls are handed out and hair is held back and Saltines are blown through like money in this house.
Then, yeah, you kind of turn into that freaky mom who sees a runny nose at a mass playdate and you bribe the kids to get in the car NOW and we'll get some ice cream on the way home.