Here's a quote from mercola.com in an article called:
The Reason for Childhood Diseases By Philip F. Incao, M.D.
"A growing body of medical research supports the commonsense idea that children who experience frequent infections and inflammations in early childhood will strengthen their immune systems and will be less prone to allergies and asthma than children who rarely experience such infections.
This idea is called "the hygiene hypothesis".
If the hygiene hypothesis proves to be correct, it will have a revolutionary impact on medical practice. We will realize that when children experience their cold and fevers, they are challenging their immune systems and developing an inner strength which will be theirs throughout life.
As with all challenges in childhood, our job as parents and healthcare workers will be to strengthen the child to meet its challenges but not to remove the challenges altogether.
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Children must experience illness to build their immune systems or they will be sick adults. They usually have illnesses frequently until the age of 7 then it drops off. Unless they are "protected" from these germs, then what happens is as soon as they are exposed to the germs later in life they get sick anyway but it's worse because adults are different. They don't rest when sick, take off work, etc...they suffer more.