In the last decade, researchers have realized that the interactions between ancient humans and Neanderthals were much more complicated than previously believed. Not only did Homo sapiens compete with Neanderthals for resources, we extensively interbred with our hominid cousins, an inter-species hookup that gave some modern humans one to four percent of Neanderthal DNA. A new study shows that humans likely gave Neanderthals something too: tropical diseases.

The study, published in the American Journal of Physical Anthropology suggests that waves of ancient humans traveling out of Africa and into the Neanderthal’s stronghold in Europe probably passed along bugs like tuberculosis, herpes, tapeworms, and stomach ulcers.

“Humans migrating out of Africa would have been a significant reservoir of tropical diseases,” study author Charlotte Houldcroft of Cambridge University’s Division of Biological Anthropology says in a press release. “For the Neanderthal population of Eurasia, adapted to that geographical infectious disease environment, exposure to new pathogens carried out of Africa may have been catastrophic.”

It was assumed many infectious diseases evolved after the development of agriculture, which allowed humans to crowd together in cities and put them in regular contact with domestic animals. But recent studies of infectious disease genomes reveal that they developed tens of thousands or millions of years earlier. Though the researchers found no direct evidence for transmission of disease between humans and Neanderthals, the paper suggests that these new timelines for diseases means its highly likely humans carried them when they migrated into Neanderthal territory.

Melissa Hogenboom at the BBC points out that researchers thought that Heliobacter pylori, the bug that causes stomach ulcers appeared about 8,000 years ago, soon after the beginning of agriculture. But H. pylori’s genome reveals it is at least 88,000 years old. A study of Herpes Simplex 2, the cause of genital herpes, shows it was transmitted to humans from an unknown hominid 1.2 million years ago.

Unlike disease transfers from Europeans to Native Americans, which led to massive epidemics like smallpox that killed millions of people in a short period of time, it’s more likely the disease transfer between humans and Neanderthals was much more localized, Houldcroft says. Because hunter-gathers lived in small bands of about 15 to 30 people, infectious diseases would have affected one isolated band at a time, weakening their overall health.

“Our hypothesis is basically that each band of Neanderthals had its own personal disaster and over time you lose more and more groups,” she tells Hogenboom. “I don't think we'll ever find a [single] theory of what killed the Neanderthals, but there is increasing evidence that lots of things happened over a period of a few thousand years that cumulatively killed [them] off.”