romans spread tuberculosis
© Marco Ravagli / Barcroft Media
What did the Roman Empire ever do for us? It spread tuberculosis
The Romans gave us roads, public toilets and the modern calendar, but we may also have them to thank for spreading a deadly disease: tuberculosis.

A genetic analysis suggests that while TB first arose about 5000 years ago in Africa, the Roman Empire was behind its more recent, rapid spread around Europe and beyond.

TB is a lung infection that, if left untreated, can cause a chronic cough, weight loss and a lingering death. By some estimates the bacterium that causes it, Mycobacterium tuberculosis, has killed more people than any other infectious disease in history.

The strain of TB that affects humans can't be carried by other animals, says Caitlin Pepperell at the University of Wisconsin-Madison. "So the evolution of the bacterium is inextricably tied to humans."

To find out its origins, Pepperell's team looked at the genetic sequences of 552 samples of TB bacteria obtained from people across most of the world. They left out North and South America as people in these continents are mainly affected by TB bacteria that arrived with the first Europeans.

Taking into account where the samples came from, and the known rate at which genetic mutations accrue in its DNA, they drew up the bacteria's family tree.

It was already known that TB bacteria can be divided into seven different families. Pepperell's team found that the last common ancestor of all these lineages first arose in Africa, probably in West Africa, between around 4000 and 6000 years ago - roughly 3000 BC.

Three of the lineages never made it out of Africa; one, for instance, is mainly found in the highlands of northern Ethiopia. But sometime before 300 BC, one lineage seems to have been carried from West to East Africa then on to southern Asia and as far afield as Papua New Guinea.

"TB spreads in closed spaces. The Romans had these gigantic bath complexes and barracks"

It isn't clear why this strain spread so successfully, but it did so at a time when ships were trading goods like shells and spices across the Indian Ocean, Pepperell told the American Society for Microbiology's Microbe conference in Atlanta, Georgia, last month.

"We think of high-efficiency global travel as the provenance of the modern era. But when we look at these patterns of migration... the bacteria appear to be zipping around the Indian Ocean, [as if it's] no big deal," says Pepperell.

Yet the most widespread strain of TB in the world today seems to have spread slightly later, around the 1st century AD. At this time, it spread from the Mediterranean region to all over Europe, Russia, Asia and even back to Africa.

"The timing is consistent with the Romans causing an incredible amount of movement and exploration around the Mediterranean," says Pepperell. "There was contact between human populations that had not had contact before."

Jared Eddy of Boston Medical Center has previously argued that the Roman Empire would probably have helped disseminate TB because it encouraged people to move into cities and towns.

In the UK, for instance, while the Romans didn't introduce the TB microbe - some Bronze Age skeletons show evidence of the disease - they did introduce conditions that encouraged it, he says. "When TB spreads, it spreads in closed spaces. They had these gigantic bath complexes and people sleeping in the same room in military barracks."