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Fingernail-size roundworms are a scourge in less economically developed countries, where they cause diarrhea, stunt children’s growth, and even kill. A new study suggests these parasites were just as common in medieval Europe as they are today, suggesting Europe’s later improvements to hygiene and sanitation proved enough to conquer them.

The new study is “extensive and well done,” says Roger Prichard, a parasitologist at McGill University who was not involved with the work. It confirms, he says, that Europe’s successful eradication efforts weren’t simply the result of naturally low parasite numbers.  

Two of the most insidious roundworms—whipworms and hookworms—infect the intestines and expel their eggs in feces, where they go on to contaminate soil, crops, and water supplies. More than 1.5 billion people are currently afflicted.

These parasitic worms, which belong to a group known as helminths, have been parasitizing humans for thousands of years. Scientists have spotted them in medieval bones and feces, even in the remains of King Richard III. Although researchers knew the parasites were there, they didn’t have a good way to estimate their prevalence.

In the new study, Adrian Smith, a zoologist at the University of Oxford, and colleagues embarked on an ambitious parasitic pilgrimage. The team collected nearly 600 soil samples from the pelvic regions of skeletons buried in cemeteries in the Czech Republic, Germany, and the United Kingdom between the seventh and 18th centuries. As a body decomposes, Smith explains, its intestines settle along the pelvic surface, making that a good place to hunt for any parasitic eggs that went to the grave with their host.

The researchers then analyzed the DNA in those samples, looking for genetic traces of two of the most common types of parasitic worms endemic in many countries today: the whipworm Trichuris trichiura, and the hookworm belonging to the genus Ascaris.

After screening the samples’ DNA, the scientists confirmed the presence of the remarkably intact parasitic eggs by closely looking at those samples under a microscope. Trichuris “looks a bit like an American football,” Smith explains, while Ascaris is a round, lumpy orb. All told, the team found that about 25% of the individuals were infected with Trichuris, and about 40% with Ascaris, and those rates stayed fairly stable over time, even into the 18th century.

That’s on par with prevalence estimates for those same parasites in parts of sub-Saharan Africa, Mexico, South America, East Asia, and other countries today, the researchers reported last week in PLOS Neglected Tropical Diseases.

Chemotherapeutic drugs developed in the 1960s can eradicate these worms, but medical records reveal that Europe was largely helminth-free by the beginning of the 20th century (though it flared up in the trenches of World War I). This suggests Europe’s improvements to plumbing, hygiene, and sanitation made the difference, Smith and colleagues conclude. And that, in turn, reinforces the World Health Organization’s calls, along with other groups,  for those improvements in other nations, he says. “These sorts of changes that happened in Europe are very powerful.”