Large tracts of Africa have 10 points shaved off their average IQ because of parasite infestation, if you get bilharziosis in early childhood it really makes you more stupid. Malaria doesn't help either, also because it causes you to miss school.

Fun fact: South Korea had a very serious schistosome problem, too. Merck discovered Praziquantel in the early 1970s, it's the only medication that really works but Merck could not provide doses at a price the country could afford. So South Korean chemists got to work and developed their own process, and it gave the Korean chemical industry a big boost. Shinpoong still produce 50 % of the world's supply of praziquantel. The WHO report is here, btw: http://apps.who.int/medicinedocs/en/d/Jwhozip48e/1.html