If its yellow out their nose, I stay home. If they are coughing up a lot, I stay at home. If they are running a temp - stay at home.

Everything else I usually see how they are the morning of, and if they are happily running around with no need for drugs - then I go.
We had a mum with Cystic Fibrosis in my first PG. She told us that this was the best way to help her avoid infections. Clear snot with no cough never bothered her. Catching a bad cold could put her in hospital so we were all aware and made sure that more than 1 of us had keys so that if one of the kids was sick we could reduce chances of her catching it.