Do I have bags? I must go and google myself! 
I see what you mean entirely.
I'd feel really let down, too - that the other parents didn't think to warn me. That must feel horrid. I'm sure it wasn't deliberate, just thoughtless perhaps.
My best friend has a child in my son's class and is very happy to send his dcs in to school the day after serious D&V type stuff. As long as they seem better.
That's not the point but some people just don't seem to understand about contagion and transmission, and think as long as their child doesn't actually vom all over you, you'll be fine sharing food/touching/whatever.
I think a mass public education campaign is in order. But I think it's stupidity rather than deliberately going out to cause trouble to everyone.
You could mention it to the mother in question, let her know how awful it was, how much trouble was caused. And say you heard another child was ill just before the party, or something like that, you wish other parents would be more considerate.
That might make her realise. Maybe it will stop her doing it again.