Back to class often means back to the doctor, as kids are exposed to more germs and more stress. "Within three weeks of school starting, we see an increase in viral upper-respiratory and gastrointestinal infections," notes Dallas pediatrician Sue Hubbard, MD, host of the radio show The Kid's Doctor. These immunity boosters can keep your child out of the waiting room.

1. SCRUB UP "Most infections are transferred by hand contact," says Chris Tolcher, MD, a clinical assistant professor of pediatrics at the University of Southern California Keck School of Medicine. He reminds his patients (and his own kids) to wash their hands with soap and warm water for as long as it take sing one round of "Happy Birthday," and then do a second chorus while rinsing. How often? before meals and snacks, and after bathroom visits.

2. EAT THE PYRAMID Feed kids meals that include whole grains, fruits and veggies, plus at least two glasses of milk and another dairy product a day. Probiotic and prebiotic foods—yogurt with live cultures, asparagus, barley, beans, berries, garlic, onions, spinach—promote "good" bacteria in the intestines and may boost immunity. "Kids tend to like white foods, but the more color on their plates, the better the nutrition," says Dr. Hubbard.

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3. CATCH ENOUGH Z's Well-rested kids stay healthier and less stressed. Sleepy students are also more likely to rub their eyes and faces, and touch their mouths to yawn--transferring infection, warns Dr. Tolcher.

4. TAKE CARE WHEN SHARING "Tell kids not to share drinks of snacks at school, as that is a sure way to catch an infection," say Dr. Tolcher. –Cheryl Solimini