The "Hygiene Hypothesis"
Kids in daycare are generally known to be at higher risk for sickness than kids who are cared for at home. It's the exposure to all those kids and their germs that's the problem -- and apparently, could also be the solution, according to some studies.
It's known as the "hygiene hypothesis" and here's the deal: While it sounds perfectly sensible to keep your environment as clean as possible and shield kids from germs, some studies also hypothesize that exposure to the very germs we try to avoid can help kids develop strong immune systems.
One recent study, done by scientist Erika Von Mutius and featured on PBS, found that children who are exposed to other kids early in life are exposed to more microbes, which might help their immune systems develop more tolerance for the irritants that cause asthma.
Of course, this is only a theory and parents should always consult a doctor regarding any germ or allergen exposure.