How did the Black Death start and what effect has it had?

New research suggests genes that helped people survive in 1300s are linked to arthritis today

Miniature from Black Death manuscript, Belgium, 14th century
The citizens of Toumai bury their dead during the black death.
(Image credit: Wikicommons)

Scientists believe the Black Death, one of the worst infectious disease outbreaks in human history, has left a mark on the immune systems of people living today.

Four DNA variants appear to have helped boost survival rates during the 14th-century plague, which was caused by the bacteria Yersinia pestis and is thought to have killed up to 60% of the western Eurasian population in just eight years.

Researchers from the University of Chicago, McMaster University in Ontario and the Pasteur Institute in Paris now say that “at least two of those variants associated with surviving the Black Death can be linked to autoimmune conditions common in modern society – including Crohn’s disease and rheumatoid arthritis”, reported The Washington Post.

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“Our genome today is a reflection of our whole evolutionary history” as we adapt to different germs, said Luis Barreiro, a senior author of the research.

What was the Black Death?

The Black Death was a 14th-century pandemic of bubonic plague, a disease caused by the bacterium Yersinia pestis. The term is a “reference to the gangrenous blackening and death of body parts, such as the fingers and toes, that can happen with the illness”, said the BBC.

While the plague has been wiped out in most of the world, “cases do continue to occur annually in rural areas of Africa, Asia and even America”, according to Oxford University, which has been working on a vaccine against the ancient illness.

There are three different types of plague: bubonic, pneumonic and septicaemic.

“If left untreated, the bubonic form has a 30 percent to 60 percent fatality rate and the pneumonic form is almost always fatal. Both bubonic and pneumonic plague can develop into a life-threatening infection of the blood called septicaemia,” explained the university.

Where did it come from?

A popular theory was that the Black Death originated in China, but this appears to have been disproved by a study published in the journal Nature in June.

“An international team of researchers now believes that the continent’s worst recorded pandemic began in Kyrgyzstan,” said Clive Cookson, science editor at the Financial Times.

They analysed DNA from human remains in two cemeteries that saw a surge of burials in 1338 and 1339. “Some tombs near Lake Issyk-Kul are inscribed with the word ‘pestilence’ in the Turkic language,” said Cookson.

Lead author Maria Spyrou, from the University of Tübingen, said: “We found that the ancient strains from Kyrgyzstan are positioned exactly at the node of this massive diversification event. In other words, we found the Black Death’s source strain and we even know its exact date: 1338.”

Her study also suggested that the animal reservoir from which the Black Death emerged was wild marmots rather than rats, although it is likely to have moved into rats and then spread to humans most often via flea bites.

“Buck-toothed and fluffy, the marmot might seem a relatively harmless rodent”, but the new research suggests “it could have been to blame for killing half of Europe”, said The Telegraph.

The newspaper explained that the plague is likely to have “seeded” in the Tian Shan region of north Kyrgyzstan into a community of Christian traders, “who then spread the disease via the Silk Road”.

The disease arrived on the British Isles from the English province of Gascony. Although it was relatively well contained in the Isles, it achieved even greater potency when the virus became airborne as it meant it was more quickly spread from human to human.

In the years between 1346 and 1353, the plague destroyed a higher proportion of the population than any other single known event. One observer noted: “The living were scarcely sufficient to bury the dead,” according to History Extra.

How did it end?

The most popular theory of how the plague ended is through the implementation of quarantines. The uninfected would typically remain in their homes and only leave when it was necessary, while those who could afford to do so would leave the more densely populated areas and live in greater isolation.

Improvements in personal hygiene are also thought to have begun to take place during the pandemic, alongside the practice of cremations rather than burials due to the sheer number of bodies.

A common myth suggests that the plagues’ third epidemic was finally wiped out in London by the Great Fire of 1666.

It’s a good story, but sadly not true, says the Museum of London. The number of people dying from the plague was already in decline before the fire, and people continued to die after it had been extinguished.

What is the Black Death’s legacy?

“A historical turning point, as well as a vast human tragedy, the Black Death of 1346-53 is unparalleled in human history,” says Ole J Benedictow at History Today.

It would take 200 years before Europe alone was able to replenish its population to pre-plague numbers. In addition to population losses, the world also suffered monumental setbacks in terms of labour, art, culture and the economy.

Another lasting effect is the new study that found that genes that may have helped people survive the Black Death have made us more susceptible to certain diseases today.

Some of the same genetic variants “identified as protective against the plague are associated with certain autoimmune disorders, such as Crohn’s disease, rheumatoid arthritis and lupus”, said Fortune. “In these sorts of diseases, the immune system that defends the body against disease and infection attacks the body’s own healthy tissues”, the magazine added.

“A hyperactive immune system may have been great in the past but in the environment today it might not be as helpful,” said Hendrik Poinar, senior author of the study.

Where does the Black Death still exist?

From 2010 to 2015, there were 3,248 cases of the plague reported worldwide, resulting in 584 deaths, said the World Health Organization (WHO).

Plague can still be found on all continents, except Oceania. There is a risk of human plague wherever the bacteria, an animal carrier and human population co-exist.

It is most common in the Democratic Republic of Congo, Madagascar and Peru, and epidemics have occurred in Africa, Asia and South America. Since the 1990s, most human cases have occurred in Africa, said the WHO.

Madagascar is known for being home to the disease, and cases of bubonic plague are reported nearly every year in the country. In 2019, a number of cats in Wyoming, USA, were discovered with the plague, prompting warnings from state officials, said Pacific Standard magazine.

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