Roundworm infection can increase the reproduction rate in Amazonian women, while hookworm infection can decrease it, a new study finds. Parasitic worms infect two billion people globally; while it's known that some parasites can cause cognitive and nutritional impairment, this study suggests that reproduction rates can also be affected by parasitic organisms. Furthermore, the authors propose an intriguing mechanism behind this correlation: the immune system. To gain a better understanding of the effects of parasites on reproduction rates, Aaron Blackwell et al. used data collected over nine years from Tsimane women in the lowlands of Bolivia, a population with an average birth rate of nine children per woman. They found that women who were repeatedly infected with hookworm were likely to have up to three fewer children in their lifetimes than uninfected women, while women infected with a species of roundworm were found to have up to two more children than those without infections. These two parasites are known to invoke different immune changes; the changes following roundworm infection happen to be reflective of those that occur during pregnancy. Specifically, as a woman proceeds through her menstrual cycle, levels of type 2 (Th2) T-cells increase and if conception occurs, this increase continues through pregnancy and helps suppress type 1 (Th1) T cells. Because roundworms are known to increase Th2 levels, and hookworms have been reported to evoke a mixed Th1/Th2 response, the authors suggest that these parasites are indirectly affecting reproduction rates by changing immune cell balances.