A: Stomach bugs, like rotavirus, can really do a number on a child's (or grown-up's) digestive system. With any stomach virus, diarrhea normally lasts for a week or so. But diarrhea longer than that should be investigated. One possible cause: Toddlers usually drink milk as their beverage of choice. But stomach viruses can break down our body's supply of lactase, the enzyme that digests milk sugar, leaving us temporarily lactose-intolerant. This is may be why the diarrhea persists, and the diarrhea should get better if you give your child lactose-free milk and dairy products for a couple of weeks. You can also give your child probiotics (supplements of "good" bacteria that help maintain a healthy digestive tract) if your pediatrician approves. If the diarrhea doesn't get better over the next week, let the pediatrician know. She may want to see your child and get a sample of her poop. Sometimes prolonged diarrhea can be caused by a parasite, which needs to be treated with medication.