About one in four people on our planet carry a latent tuberculosis infection, and the disease is one of the top 10 causes of death worldwide. 

A new study has traced the genetics of the germ that causes TB - Mycobacterium tuberculosis - to help explain the disease's broad reach, one that has its roots in Africa and branched out with the expansion of the Roman Empire's barracks and bathhouses.

Research led by the University of Wisconsin-Madison reveals that when Rome emerged as a superpower in the Mediterranean, it gave at least one variety of the tuberculosis pathogen an easy ride.

Tuberculosis most commonly infects the lungs; thankfully, it usually stays latent without producing any symptoms, remaining hidden while also not being infectious.

But in a proportion of people carrying the latent infection, Mycobacterium tuberculosis will at some point engage in a heated war with the immune system, where it avoids destruction and turns the tables by replicating inside the defending white cells.

The results can be ghastly, leaving graveyards of dead white cells called tubercules in the lungs. These granules cause irritation, giving rise to the bloody cough tuberculosis is historically known for.

To better understand how the disease could spread in the future, it helps to know how it got here in the first place. Which is precisely what population geneticist Caitlin Pepperell has discovered by looking at the bacterium's genome.

"What I wanted to do was to embed the genomic data into a sensible historical context," Pepperell explained in a session on paleomicrobiology at last month's American Society for Microbiology's Microbe conference.

Seven major tuberculosis strains have been identified across the world, all linked to a human disease that first appeared some tens of thousands of years ago in Africa.

It took the frequent association of human settlement for the bacterium to really take hold, and from there on it has left its mark on human populations ever since.

Pepperell's genomic data came from more than 550 samples of tuberculosis bacteria gathered from modern day populations across Africa and Eurasia.

Comparisons between their sequences were used to show how the different strains connected into a phylogenetic tree.

By using the reliable tick of mutating segments of DNA, Pepperell and her team could put a date on the steady creep of each strain as it made its way from Africa to the far reaches of the planet.

They found that the last time all of the strains were one, big, infectious family was between 4000 and 6,000 years ago in Africa. While three of the seven lineages didn't spread very far, others exploded.

Tellingly, the dates of its branching roughly aligned with significant events in the rise and fall of civilisations: one of the most widespread strains went gangbusters around the 1st century AD.

"The timing is consistent with the Romans causing an incredible amount of movement and exploration around the Mediterranean," Pepperell told Clare Wilson at New Scientist.

"There was contact between human populations that had not had contact before."

Not all of this contact was necessarily with infected Romans - but it didn't need to be. Barracks for soldiers fighting for or against the Roman military machine could have been involved, not to mention the spreading fashion of getting the clan together into a warm, wet room for a good wash down.

Several years ago a study led by the National Museum of Natural History in Paris found a strain of tuberculosis called the Beijing Lineage emerged near north-eastern China some 6,600 years ago, coinciding with the development of rice farming in the area.

Pepperell's analysis found a similar pattern describing the movement of the strain, which they also noted was remarkably similar throughout the country, possibly as a result of a history of isolationist policies.

The fact human diseases spread along trade routes and coincide with migration and military movements isn't exactly surprising. What results such as these do is flesh out what we know about cultural events and behaviours throughout the ages, helping us understand the limits of how diseases can spread.

Untreated, roughly half of those who get active tuberculosis succumb to its effects, which left a terrible toll as communities in the past increasingly moved from the country into cities.

It's not pretty, and with the appearance of drug-resistant strains, it could even become a pandemic we will need to keep an eye out for in the future.

Let's hope that our growing knowledge of the disease will eventually mean the only studies we see are those of its past.

The research can be found on the pre- peer review website bioRxiv.org, available for researchers or anybody who's interested to dive into and discuss.