A group of international scientists studying how a drug-resistant strain of tuberculosis (TB) managed to spread around the world has discovered the Industrial Revolution and the collapse of the Soviet Union could be to blame.
TB is a bacterial infection that spreads from person to person when tiny droplets from coughs or sneezes of someone infected with it are inhaled.
Led by Thierry Wirth, an evolutionary geneticist at the National Museum of Natural History in Paris, the researchers studied almost 5,000 samples of Mycobacterium tuberculosis from patients in 99 countries.
The researchers also sequenced a full genome of 110 of the samples, drawing up a detailed family tree.
The results, entitled "Evolutionary history and global spread of the Mycobacterium tuberculosis Beijing lineage", are published in the journal Nature Genetics.
Where did drug-resistant TB come from?
Although it is known that M tuberculosis probably emerged around 40,000 years ago in Africa, the disease did not truly take hold until humans started farming practices and grouped together in settlements 10,000 years ago in the Fertile Crescent (a region stretching from Mesopotamia to the Nile Delta).
The drug-resistant strain MDR TB is known as the "Beijing lineage", which was first identified in greater Beijing in the mid-1990s. The strains in this lineage are drug-resistant to drugs able to vanquish other types of TB.
From their study, the researchers discovered MDR TB spread out of East Asia to the Middle East due to trade on the Silk Road.
The branch of the strain that currently circulates in the Pacific could have been brought over by Chinese people emigrating to the Pacific Islands in the 1850s, while another branch of the strain came to former Russian republics of Central Asia through the arrival and dispersal of Chinese immigrants in Kyrgyzstan, Kazakhstan and Uzbekistan in the 1860s and 1870s.
Meanwhile, it is thought the rise of large urban populations in European cities such as London during the Industrial Revolution (between 1820 and 1840) probably led to MDR TB being brought over and then spread over Europe.
The rise of tuberculosis today
In the 20th century, there was a spike in MDR TB after the First World War, when further urbanisation together with influenza pandemics would have made people more susceptible, as the condition takes advantage of those who are not well.
"If people are weak or tired or lack food, this weakens the body, making them more susceptible," Wirth told New Scientist.
TB numbers then dropped due to the rising availability of antibiotics in the 1960s, only to make a comeback in the early 1990s after the collapse of the Soviet Union, when the Beijing lineage of TB became even more infectious.
The UK currently has the second-highest rate of the lung disease among Western European countries and in 2013, there were 7,290 cases reported in England, 68 of which were drug-resistant TB cases.
While regular cases of TB can be cured within six months, patients suffering from MDR TB are required to take several different drugs at the same time for one to two years, and they have side effects such as causing liver and nerve damage.
The strain currently being seen in London can be traced directly back to Russia and Eastern Europe.