Necessary infections

Some kinds of herbivore mothers deliberately feed their children mom's feces. Their guts need the bacteria to digest food properly. Of course, sometimes parasites hitch rides, but that can't be helped. As long as the parasites don't kill the baby animal before it has an opportunity to infect the next host, the parasites can survive.

Human brains do not work without the necessary infections of memes -- primarily supplied by the parents, but also from teachers, peers, and so on. Of course, sometimes parasites hitch rides. Unlike the herbivores, we can deliberately weed out the unnecessary and harmful memes.
(Or can we?)