Parasites are pretty horrifying, but the reality is that approximately 60 percent of us are infected by at least one – and according to a growing body of evidence, that may not be as bad as you think.

New research has found that indigenous women in Bolivia have significantly more children when they're infected with roundworm, suggesting that the parasite may be making them more fertile. Meanwhile, hookworms appeared to have the opposite effect.

But before you start chugging down dirty water in the hopes of boosting (or hindering) your own fertility, let's be very clear here – the study simply showed a correlation between parasites and number of children, and didn't provide any insight into how the intestinal worms could affect fertility. But it wouldn't be the first time that parasites have been shown to pass some benefit onto their human hosts. 

Earlier this month, scientists in Australia announced that they were about to infect 40 celiac patients with hookworm, after early trials found that patients who couldn't stomach gluten were suddenly able to eat pizza, bread, and pasta when infected with the parasite. There's also evidence that certain parasites can help heal wounds and potentially even treat lung disease.

And now it appears even our fertility might benefit from the occasional intestinal worm. In the new study, a team of anthropologists from the University of California, Santa Barbara, studied a group of almost 1,000 indigenous Bolivians, known as Tsimané people, for nine years.

The Tsimané are often infected with roundworms and hookworms as a result of contaminated food and water, but most of them show few, if any, symptoms of the infection. But when the team looked at the data, they found a surprising correlation. 

"We found that different species of helminths – a family of parasitic intestinal worms – could have either positive or negative effects on the timing of a Tsimané woman's next pregnancy," said lead author Aaron Blackwell. "Hookworm infection tended to increase the length of the intervals between births and that was consistent across all ages. But younger women infected with roundworm had shorter birth intervals."

Women with roundworm also had their first child younger, while those with hookworm had it later, and over a lifetime, that made a big difference.

The researchers found that women who were "chronically infested" with roundworm would have an average of two more children than those never infected (the average Tsimané woman will have 10 children), and those with hookworm would have three fewer. 

So how could intestinal worms be affecting fertility? Further research needs to be done to investigate the mechanisms, but the researchers told Quartz that they think it all comes down to the immune system. 

In healthy women, ovulation triggers changes that calm the immune system down and force it to produce fewer type-1 cells, which attack foreign bodies directly, and makes it produce more type-2 cells, which hang back and produce antibodies.

It's this immune suppression that allows a woman's body to tolerate 'invasive' bodies, such as a partner's sperm. It also stops the immune system from destroying a fertilised egg before it has a chance to attach to the uterine lining.

Interestingly, roundworms are known to trigger the same immune response as ovulation, while hookworms have the opposite effect, and increase the number of aggressive type-1 cells in the bloodstream. 

"So it may be that when women have roundworm, their immune systems are already less inclined to attack an embryo, and so their embryos have a higher likelihood of surviving the first stage of pregnancy," as Quartz explains.

"Although we don't know the precise mechanism behind these results, our findings are still compelling and suggest that immune modulation – via our 'old friends' the intestinal worms – can have far-reaching effects on the body," said study co-author Michael Gurven

Further research is now needed to investigate exactly what's going on, but if parasites can help our immune system 'prime' women's bodies for a baby, it could have big impacts for women who struggle to conceive. 

"These results may also have implications for fertility in developed populations, where many fertility problems are connected to autoimmune disorders," said Blackwell.

The research has been published in Science.