I was listening to a piece on the BBC about this. There's been a recent study with women in Bolivia. Parasitic worm infections--roundworm vs. hookworm infections--one boosted fertility; the other lowered it. The study might prove useful in future innovations of fertility drugs.
https://www.washingtonpost.com/news/to-your-health/wp/2015/11/20/this-parasitic-worm-appears-to-boost-female-fertility/