A study suggests that infection with a species of worm increases the fertility of women. A study of 986 indigenous women in Bolivia indicated a lifetime of Ascaris Lumbricoides, (a type of roundworm) infection alters the immune system to make it easier to become pregnant. But while Ascaris Lumbricoides increased fertility in the nine-year study, hookworms had the opposite effect, leading to three fewer children across a lifetime.

About 70% of the population has a parasitic worm infection in Bolivia. Up to a third of the world's population also lives with such infections. Ascaris Lumbricoides is the giant roundworm of humans, growing to a length of up to 35 cm (14 in). It is one of several species of Ascaris. An ascarid nematode of the phylum Nematoda, it is the largest and most common parasitic worm in humans. This organism is responsible for the disease Ascaris, a type of Helminthiasis and one of the groups of neglected tropical diseases. An estimated one-sixth of the human population is infected by A. Lumbricoides or another roundworm. Ascaris is prevalent worldwide, especially in tropical and subtropical countries.

Experts say that the findings could lead to "novel fertility enhancing drugs".