And parents know that the first colds and other viruses can't be far behind.
Experts at Nemours duPont Hospital for Children have strategies for cutting down on those ailments.
With 240 children under age 5, all year round, there's a lot of potential for germs to spread at the Nemours Child Development Center.
But staffers take a host of steps to slow them down.
"First one is handwashing. that is the biggest line of defense for us," says center director Lauren Francis.
And kids learn how to do it right, from the very beginning.
"For 20 seconds under warm running water with soap, drying with paper towels right away.," says Francis.
All the utensils are used just once.
And gloves are a must for teachers handling food, changing diapers, or taking care of sick kids.
Once meal-time is done, children clean up after themselves.
"We have placemats for every child. we'll spray them down with a soap and water solution," says Francis.
And the table itself is cleaned with disinfectant - several times a day.
Down the hall, infants spend the day on the floor, working on strengthening tummy muscles.
So the floor needs to be clean.
Francis notes, "Any visitors that come into the classroom, whether it's parents or our teachers, they always have shoe covers."
And any toys that children put in their mouths go into a dirty toy bin.
As soon as the center knows a child has been diagnosed with a contagious illness, it's posted outside the classroom.
And the notice includes what symptoms to look out for and what precautions to take.
Pediatrician Dr. Jonathan Miller says repeated infections are normal for young children, especially in the first year or so of being in a group setting.
"The average child in their toddler years get like 8 to 10 viral respiratory infections. 8 to 10, right?" Dr. Miller.
He says he's been through the day care and school germ cycle with his own children.
So Dr. Miller says being a germ-buster at school AND at home can help.
Francis says Nemours Center parents are urged to follow the same steps that school staff does,
Dr. Miller also advocates another major preventive step -
"The single best thing you can do to prevent infections getting transmitted is to get vaccinated for everything that's recommended," he says.
Francis gives these additional tips:
* Wear disposable gloves for all diaper changes, or whenever caring for a sick child. And wash your hands before and after.
* Cribs are disinfected once a week or more often if it's needed
* Wash crib sheets frequently
* Floors are swept in between meals, and cleaned daily
* Classrooms are tidied in between classes by teachers, and daily by a cleaning crew