Parasites (round worms) in Africa boost fertility in women since they seem to suppress imuno-reactions paving the way for sperms. (see Aaron Blackwell University of California)

Probably one reason why their fertility is with 4,7 children/women (2015) higher than 1,6 in EU. http://de.statista.com/statistik/daten/studie/1724/umfrage/w...