Answer: Do not send your child to school or day care for at least 3 days (72 hours) after symptoms have stopped (48 hours minimum).
Since people are still contagious with stomach bugs for 3 days after symptoms have stopped, ideally, your child needs to be well for at least 3 days before you send them back to school or day care. That means that it has been 72 hours since they last vomited or had diarrhea. If they are not eating normally and don't have normal poop, they are still sick and are still contagious.
Parents are often confused about how long to keep a child home from school after a vomiting illness. It is common today for parents to send their kids back to school or day care as soon as they are feeling better. This is the main reason the stomach flu (viral gastroenteritis) spreads like wildfire. If you send them back to school only 24 hours after their last episode of vomiting or diarrhea, they will still be contagious and they may not be done having vomiting or diarrhea. There is often a delay between the vomiting and diarrhea segments of the stomach flu. A child may be up all night throwing up, be fine the next day, and have the diarrhea start on the following day. I have personally experienced a 24-hour period of calm between the vomiting and diarrhea. It would be a terrible idea to send your child to school during that time.
48 hours is the absolute minimum amount of time to wait to be sure that your child is done being sick. Once my son had a virus where he vomited several times on Tuesday, was better on Wednesday, but vomited again on Thursday. He also had a diarrhea illness where he had went 40 hours between explosions. I thought he was fine until another one hit. During a recent stomach bug that traveled through our house in Jan/Feb 2012, my son Jon, vomited 3 times on Sunday. He had no symptoms on Monday but he hardly ate anything. Most other people would have sent him to school on Tuesday. I didn't because he wasn't eating. If the kid is not eating normally, they are still sick. Tuesday afternoon, he complained of bad tummy pains and took a nap. When he woke up he had one diarrhea explosion. It had been just over 48 hours where he didn't have any symptoms at all. How terrible would that have been if it happened to him at school?
Keep your child home until they are good and well. 48 hours is the minimum amount of time it takes for you to be sure your child is actually done having symptoms. 72 hours is the minimum amount of time that they will be contagious. If you have an older child and you can only keep him home for 2 days because he has a big test, make sure he understands the importance of hand washing.
Do not take your child to birthday parties, indoor playgrounds, playgroups, or swimming pools until they have been well for 2 weeks. In a swimming pool, tiny traces of virus filled feces are whisked away from you child’s bottom and float in the pool until another unsuspecting child swallows a mouthful of water. 48 hours later that child is sick. If parents just kept their kids home for a few days after the stomach flu, we all wouldn’t have to get it so often!
--Annie Pryor, Ph.D.