Back to school (2012 Robert D. Barnes)

New knowledge? Sure—and new germs. Send the kids back to school, and wait for the bugs they'll inevitably bring home. "Prevention can be a little difficult, but it's the same as your mother taught you: Keep your hands away from your face and wash your hands," says pediatrician Gordon Schutze, who serves on the American Academy of Pediatrics' committee on infectious diseases.

All it takes is one kid to cough or sneeze in the classroom, and suddenly his peers are sick too, because they inhaled the infected respiratory droplets. Or a kid with diarrhea forgets to wash his hands after using the bathroom and proceeds to infect everything he touches. That's why proper hand washing is so important, says Albert Martinez, a pediatrician based in San Diego, Calif., who describes it as "the best prevention strategy." (Teach kids to use soap and water, and scrub vigorously for 20 seconds—the time it takes to sing the "Happy Birthday" song two times.) Arm your child with alcohol-based hand sanitizer to keep in his desk, too. It's smart to use before eating lunch, and after using a shared computer mouse, pencil sharpener, water fountain, or other communal object. And though kids sometimes like to sample each other's lunches, remind yours not to share water bottles, food, or other personal items.

Beware: These common back-to-school contagions may be coming home with the homework:

Hand, foot, and mouth disease. The viral infection causes mouth ulcers and tiny blisters on the hands and feet. "There's been an increase lately," Martinez says. Although it's moderately contagious, it's usually not serious. There's no specific treatment, but practicing good hygiene—such as frequent and thorough hand washing—can keep your little one safe.

Colds. There's no such thing as "cold season." Colds can strike at any time of the year, and are caused by more than 20 different viruses. Symptoms include congestion, a runny nose, headache, cough, sore throat, and tiredness. Typically, a cold-ridden kid is contagious for two to three days. The best medicine? Lots of fluids and plenty of rest.

Respiratory viruses. These are "very contagious," Martinez says. Particularly worrisome: Respiratory syncytial virus, or RSV, which sends at least 2 million kids under age 5 to the doctor or hospital each year. Symptoms include shortness of breath, a "seal bark" cough, fever, stuffy nose, and wheezing. It's most common from late fall through early spring.

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Strep throat. Ouch. Strep brings fever, stomach pain, and red, swollen tonsils. Since strep-causing bacteria migrate to the nose and throat, sneezing, coughing, and shaking hands can spread it from person to person. Strep requires antibiotic treatment, but kids typically recover within a few days. If it's not treated, it can trigger scarlet fever—all the symptoms associated with strep, along with a scarlet-colored rash that commonly appears on the neck, chest, armpits, elbows, groin, and inner thighs.