S.C.
I believe the rule is that they're no longer contagious if they're on antibiotics for 24 hours. You should be able to contact your pediatrician or pharmacist to be sure. Also, don't be afraid to ask your sitter what she did to decontaminate all the toys and such. Chances are your child has already been exposed though. Generally speaking, we are contagious before we show symptoms of any infection.