More than 6000 years ago, a small child in ancient Mesopotamia went wading in a nearby stream. He or she might have been bathing, playing, or merely cleaning up after answering nature’s call on the stream’s bank, a common practice in the days long before toilets. But the wader was out of luck; lurking in the water were the treacherous larvae of a parasite called Schistosoma.

The larvae burrowed through the child’s skin and passed through the liver, before eventually settling in the intestines or urinary tract and growing into full-grown flatworms as long as 1 to 2 cm. In an acute case, these worms would have caused fever, bloody stools, and an enlarged liver. In a chronic case, the infected child would have grown anemic and wasted away, making him or her vulnerable to liver damage and bladder cancer.

It is hard to say which symptoms the ancient Mesopotamian child suffered because no soft tissue remains on his or her skeleton today. The disease did leave a trace, however. The mature worms in the child’s pelvis laid eggs, and one of them stayed buried for thousands of years. Now, a team of researchers, led by paleopathologist Piers Mitchell of the University of Cambridge in the United Kingdom, has unearthed the egg in a grave in the Tell Zeidan archaeological site in modern Syria.

Before the Tell Zeidan discovery, the oldest confirmed case of schistosomiasis was a 5200-year-old mummy in Egypt. The egg at Tell Zeidan is the first confirmation that the infection existed in Mesopotamia as well. “This finding is interesting because it further establishes the antiquity of the human-parasite association at an earlier date than is currently known,” says Charles Faulkner, a parasitologist from Lincoln Memorial University in Harrogate, Tennessee, who wasn’t part of the study.

To find the schistosome egg, Mitchell and his team collected sediment from the pelvic area of 26 skeletal remains from a cemetery of the Ubaid people, an ancient Mesopotamian farming culture. The researchers then mixed the sediment with water and passed it through a microsieve. When they examined the sieved particles from one grave under a light microscope, they spotted the 132-micrometer-long schistosome egg, the researchers report online today in The Lancet Infectious Diseases.

Mitchell and his team also collected sediment from near the head and the feet of all skeletons as control samples. They found no eggs in these regions, which suggests that the schistosome wasn’t the result of modern contamination at the gravesite.

The Syrian find throws light on the role of agricultural technology in increasing the spread of infectious disease among humans, Mitchell says. “A lot of different parasites—roundworms, hookworms, whipworms—find it difficult to infect you if you are moving a lot of time,” like nomadic hunter-gatherers do. This is because the populations of nomadic groups remain small, they don’t stick around long enough to contaminate any one water source, and they tend not to keep domestic animals such as sheep and dogs, which can be sources of parasites. Once humans turned to agriculture, however, they settled down, and their populations grew past the critical thresholds that infectious diseases require to sustain themselves.

The places humans choose to settle may also have brought them in closer contact with parasites. The Ubaid people, who lived in Tell Zeidan between 6500 and 6000 years ago and buried their dead in the cemetery Mitchell studied, are known to have pioneered the use of irrigation to grow food on their arid land. Living near and working in those canals could have put humans right in the schistosome parasite’s path, allowing it to jump from its temporary hosts—freshwater snails—to people’s intestines, just as it continues to do today in Africa, Asia, and South America.

Still, Scott Lawton, a parasitologist from Kingston University London, cautions against drawing too many conclusions about the source of the 6000-year-old infection based on the finding of a single egg. “A single egg simply tells us that the particular person was infected with schistosomiasis; so, drawing any major conclusions regarding route of infection is difficult. It could have been through irrigation, it could have been through natural water ways, it may have even been an infection picked up from travelling elsewhere in the Middle East or North Africa,” he says. “There is certainly more work to be done to disentangle the causes of infection in the Syrian gravesite.”

*Correction, 20 June, 10:53 a.m.: An earlier version of this article incorrectly quoted Piers Mitchell as saying that pinworms, hookworms, and tapeworms find it hard to infect people when they are moving a lot of the time. His statement was about roundworms, hookworms, and whipworms. The article has been corrected to reflect that.