She had kindly offered to help set up (toys & balloons in a local hall), put food out, etc but rang me that day to say she had a sick bug. I told her she needn't come if she was ill and that I had lots of help already, she should stay home, rest and get better
She turned up anyway - 30 mins after the start so all setting up was done, announced she was still feeling really ropey and that her DP now had it and was in bed (but said she wasn't contagious any more 
)
I was annoyed and told her as she lunged in for a hug (perhaps a little too sternly as I was busy with sorting the party) not to kiss me or my kids and to keep out of the kitchen and away from all the food. She protested that she was fine and not contagious and wanted to help but I replied that I did not want a 'sick bug' passing around, especially as I had done all the food! I'm not normally precious about germs for myself and my own kids, but when I'm hosting and feeding 20 kids + parents I would feel a little responsible should they all get ill.
She sulked in a corner with a cup of tea for a while, telling anyone who sat near her how awfully ill she was then just as we started clearing up she made some excuse and left.
Aarrgghh