I'm rather cross about this, but will try not to rant too much.
DD has recently been diagonsed with Impetigo, when we first noticed the spots we thought it may be Chicken Pox so we texted all the mums of the babies we'd seen that week to let them know, so once she was diagnosed with impetigo, we then texted them all again, to let them know.
It now turns out that one of the mums who attends one of the play groups didn't bother to tell anyone that her DS had imetigo (which she knew about at the time), and thinks it's such a coincidence that our DD caught it the same week as her DS (she was playing with him a few days before her spots started to show - when he already had it). 
Anyway I get home last night and DD now has conjunctivitis, again another mum has decided to take her DD to a party with loads of other children, some as young as 4 weeks and let them spread this.
I appreciate kids will get ill and will pick things up from each other, I know it's all part opf growing up and developing their immune system.
But FFS how about either making the sensible choice and saying "my DD has XYZ, so we won't be attending" or at least tell the other parents so they can make the choice as to whether or not to expose their kids to it.