The reason poli took off in the US is because of the same thing, kids were not exposed to the virus as infants on their mother's milk which would have protected them while they made their own antibodies to the virus. When the kid was exposed to the virus later in life they had no protection and got sick. Perhaps exposing children to mild parasites so that their immune system has something to play with is the answer. Also not keeping everything psychotically clean all the time, let the kid play in the dirt or something. My siblings and I were raised tough and did not miss much school, my cousins were raised in a more sterile environment and seemed to get sick a lot more.
~~Nonpathogenic microbes are our friends~~