Sewer systems have many fine selling points, only one of which is that they tend to reduce the spread of disease.

Or at least in theory—according to a new paper published in the International Journal of Paleopathology, that might not have worked quite as well in practice. That's the conclusion a team of archaeologists interested in public health drew from studying a Roman town called Sagalassos in what is now Turkey.

Within the ancient town, which came under Roman control in 25 B.C., the archaeologists identified a bath complex, and within it a latrine rigged so water flowed through it and washed the waste into the local sewer. So the scientists collected samples to bring back from the lab. Thanks to the room's later use as a compost facility, they had to sort through the poop samples to eliminate those of herbivores. That left them with five human samples, which the team believes date from between 100 and 500 A.D.

In every single one of those samples, the scientists found the eggs of a type of parasite called Ascaris, a roundworm that is rare in the U.S. but still infects more than 800 million people on Earth today. Individuals catch the parasite by eating its eggs, which then hatch and develop into worms that release new eggs into the infected human's poop. Today, we have fairly effective drugs that can kill the worms. The archaeologists also identified other infections, including Giardia duodenalis, which causes diarrhea (and which more than 1 million Americans catch each year).

Both Ascaris and Giardia infections can be transmitted without causing any symptoms, so the new paper isn't necessarily evidence that the ancient residents of Sagalassos were suffering. (The authors also note that there's no way to identify how many people the samples came from.)

Ascaris eggs are relatively common archaeological finds, with several samples dating back as far as 3,000 B.C. and spanning from France to South Africa to America. But before this paper, the oldest identified Giardia infection in Europe was dated to between 600 and 900 A.D. in Switzerland. That means this new paper pushes its presence back a couple of centuries, although much older samples have been found in the Americas.

The authors suggest that the infections could have been caused by unsafe food preparation like not washing hands, by using human waste as fertilizer, or by sick people bathing with healthy people. Other scholars have suggested that despite the Roman reputation for engineering prowess, their sewers simply weren't designed very well, and possibly clogged regularly. To really make a difference with parasites, engineering and public health have to go together.