By A.D. 400, Rome could count 11 magnificent imperial baths, decorated with mosaics, marble and sculpture, and more than 800 smaller spa complexes.

The ruins of those facilities have long impressed on us a view of Rome as a civilization of elevated health and hygiene.

The reality is that despite those amenities, the Romans were as riddled with parasites as their predecessors. Fish tapeworm, which appears to have been endemic to a fermented fish stew that traveled the empire in clay jars, was more widely found in Roman times than before. Poor sanitation parasites like roundworm were just as common as before.

Evidence of head lice and their eggs have been discovered on wooden combs from the Roman period in Israel; excavations of Roman-era Britain have turned up “large numbers of human fleas,” and what appears to be a bed bug.

“Despite their introducing sewers, public toilets, clean drinking water, we don’t see any drop in those parasites when you change from the Bronze Age and Iron Age to the Roman Empire,” says Piers Mitchell, an ancient disease researcher at Cambridge University. Mitchell’s report, published in Parasitology, tries to posit what might have gone wrong in Rome.