http://www.bbc.co.uk/news/health-34857022 "Infection with a species of parasitic worm increases the fertility of women, say scientists. A study of 986 indigenous women in Bolivia indicated a lifetime of Ascaris lumbricoides, a type of roundworm, infection led to an extra two children. Researchers, writing in the journal Science, suggest the worm is altering the immune system to make it easier to become pregnant. Experts said the findings could lead to "novel fertility enhancing drugs". Nine children is the average family size for Tsimane women in Bolivia. And about 70% of the population has a parasitic worm infection."