The belief that children's immune systems need to be exposed to viruses and bacteria in order to strengthen them, but that children are overprotected from this exposure, is called the

This belief is called the hygiene hypothesis which is not completely valid and tested as truth and applicable to all cases. This states that once a child is exposed more to viruses and bacteria, the child becomes immune to sicknesses. Of course, this takes all the factors to play to determine the index of a child's immunity to illnesses.