http://www.newsweek.com/why-do-some-nations-have-lower-iq-scores-74797
Diarrhea makes people dumb.
The Idea
The brain, say author Christopher Eppig and his colleagues, is the “most costly organ in the human body.” Brainpower gobbles up close to 90 percent of a newborn’s energy. It stands to reason, then, that if something interferes with energy intake while the brain is growing, the impact could be serious and longlasting. And for vast swaths of the globe, the biggest threat to a child’s body—and hence brain—is parasitic infection. These illnesses threaten brain development in several ways. They can directly attack live tissue, which the body must then strain to replace. They can invade the digestive tract and block nutritional uptake. They can hijack the body’s cells for their own reproduction. And then there’s the energy diverted to the immune system to fight the infection. Out of all the parasites, the diarrheal ones may be the gravest threat—they can prevent the body from getting any nutrients at all.