Parasitic worms bore into our organs, steal our nutrients, and sup on our blood—but their effects aren’t all harmful. A new study of people living in the Amazon rainforest suggests that certain intestinal worms increase the number of babies women give birth to.

“This is a very original study,” says parasite immunologist Rick Maizels of the University of Edinburgh, who wasn’t connected to the research. “I think it’s going to spark a lot of other investigations” on the reproductive impact of the worms.

More than 1 billion people are infested with intestinal worms, mainly in tropical areas with poor sanitation. One of the most common is the giant roundworm Ascaris lumbricoides, which can grow to 36 centimeters (14 inches) in length. Giant roundworms reside in the small intestine and swipe a portion of their host’s food. Other worms, such as the hookworms Ancylostoma duodenale and Necator americanus, are tiny vampires. They puncture the lining of the intestine and drink their host’s blood.

For all their harmful and icky habits, parasites have a lot in common with a fetus in the womb. The immune system regards a parasite and a fetus as interlopers, so both need strategies to foster what researchers call immune tolerance. Parasites can trigger some of the same immune changes that occur during pregnancy—for example, stimulating regulatory T cells, which quell immune attacks.

Because of these similarities, human biologist Aaron Blackwell of the University of California, Santa Barbara, and colleagues wondered whether parasitic infections could pave the way for pregnancy. The researchers tried to answer the question by analyzing data on the Tsimane people who live in the Amazon rainforest of Bolivia.

The roughly 16,000 Tsimane survive mainly by hunting, fishing, and raising crops such as rice and plantains. Their homeland is prime parasite country. About 15% to 20% of them harbor Ascaris, and 56% of them carry hookworms. Infected women in the study usually were unaware they were playing host to the parasites, Blackwell says.

The only health effects the researchers could detect among nearly 1000 Tsimane women were in individuals carrying hookworms. They had a slightly smaller body mass index and lower levels of the oxygen-carrying protein hemoglobin in their blood. Hookworms were also detrimental to fertility. They increased the age at which Tsimane women first gave birth and stretched the amount of time between pregnancies. As a result, the team calculated, a woman with hookworms would bear three fewer children in her lifetime than would a woman lacking the parasites. For Tsimane women overall, however, fertility is not a problem, as they give birth to an average of nine kids.

In contrast, the giant roundworm Ascaris was a boon for reproduction. It shortened the time between pregnancies and reduced the age at which women first give birth. A woman infected with Ascaris would bear on average two more children in her lifetime than would a woman free of the parasites, the researchers report online today in Science. “It’s a little bit counterintuitive,” Blackwell says.

By tweaking the immune system, Ascaris worms reduce inflammation and thus might promote conception and implantation of the embryo in the womb, the team speculates. Hookworms, in contrast, aren’t as good at inhibiting inflammation, and their blood sucking and nutrient stealing could overwhelm any reproductive benefits they provide, the researchers suggest. Blackwell and colleagues are now analyzing blood samples from the women to determine which cells and immune molecules the worms alter.

Other studies have revealed that bacteria living in our bodies are essential for pregnancy, notes reproductive immunologist Gil Mor of Yale School of Medicine. But the suggestion that “worms may be affecting reproduction, even improving reproduction, is quite surprising,” he says. The study highlights the fact that “the state of the immune system is of crucial importance to successful reproduction,” says Norbert Gleicher, a reproductive immunologist at The Rockefeller University in New York City.

Because the immune changes triggered by parasites can suppress allergies, asthma, and autoimmune diseases, several clinical trials have tested whether infecting people with worms soothes the symptoms of these conditions. Researchers doubt that Ascaris will ever become an infertility treatment, however. “I would never give my patients that parasite,” Mor says. But the results could still lead to new therapies, Gleicher says. “If this [finding] is true, we could develop an immunization that produces the same kind of immune response as does roundworm infection.”