Next, a lump -- sometimes as small as pea, other times as big as an apple -- protruded from their skin. Then, as the disease spread throughout their body, they coughed up blood

Finally -- for many of them -- came death.

That was how people hundreds of years ago described the Black Death , which began sweeping across Europe in the 14th century, killing up to 60% of the continent's population in one of the worst pandemics in human history.

Today, many of us think of the plague as something confined to the history books -- a grim symbol of the medieval period, before doctors knew about the existence of viruses or bacteria.

But this month, three people in China were diagnosed with two different forms of plague , highlighting that while the plague is not as serious an issue as it once was, it's also not entirely a thing of the past.

Neither is debate about the cause of the disease, how it spread, and even where it came from.

Plagued by questions

For a disease that has impacted humans for centuries, there's still plenty we don't know about the plague.

Humans have been hit by three major plague pandemics over the past 2,000 years, resulting in nearly 200 million deaths . The first pandemic was in the 6th century , during the reign of Byzantine emperor Justinian I. The second -- which was known as the Black Death -- swept through medieval Europe, starting from the 14th century. The third pandemic began in China in the 19th century, and spread to other parts of Asia and the United States.

In the Middle Ages, many thought the disease had been sent by god as punishment for their sins. By the 20th century, scientists were pretty sure that all three pandemics were caused by the bacteria Yersinia pestis , which is found in small mammals and fleas. They knew that there were a number of varieties of Yersinia pestis, the most common of which are pneumonic and bubonic -- the type that causes large sores.

14th-century plague of Florence as described by Giovanni Boccaccio.

But starting in the 1970s and 1980s, historians and biologists began pointing out that the second pandemic didn't act like the third pandemic in a significant way: it killed many more people. That prompted people to posit that another disease had caused the Black Death, said historian Winston Black, who is writing a book that busts a number of plague theories.

"They're often called the 'plague deniers' -- they're denying that the medieval Black Death was the bubonic plague," Black said. "They've proposed anthrax, (and) something like an early Ebola."

The turning point came in the 2000s, when scientists developed the ability to extract ancient DNA -- including from medieval skeletons.

When scientists analyzed the skeletons of plague victims, they found fragments of Yersinia pestis, said Black. But that only led to another question: if the disease wasn't genetically different, then why was the second pandemic so deadly?

In the past, that's been attributed to the poor hygiene and close living quarters of people during the medieval period. But Black says that still doesn't completely explain it, as others have lived in similarly bad conditions and not experienced such a rapid and deadly plague.

And there are other questions and misconceptions that remain over the Black Death.

Although the nursery rhyme "Ring Around the Rosie" is widely thought to be about the plague, Black said that was an incorrect theory created decades after the song was first sung. And the popular notion that doctors wore beaks -- supposedly to protect them from infection -- during the Black Death was also wrong, Black said -- the mask wasn't invented for hundreds of years after the second pandemic.

A world map that is believed by some to have been compiled by Zheng He(1371-1435), China's most famous navigator.

About a decade ago, some scientists argued that the plague could have originated in East Asia over 2,600 years ago. The second pandemic could have started in China, they said, and been brought to Europe through the Silk Road , an ancient trade route that connects China to Europe. They also posited that the disease could have been brought to Africa by Zheng He , a Chinese explorer who traveled around the world in the 15th century , and who has drawn comparisons with Italian explorer Marco Polo.

But scientists have since found DNA evidence that the plague could have existed much further back than previously thought -- there's evidence it existed in Europe some 5,000 years ago

And the idea that the second pandemic, the Black Death, could have started in China is unlikely, Black said.

DNA evidence extracted from the skeletons of medieval plague victims, and genetic analysis of the bacteria, suggest that the outbreak probably originated in central Asia, and moved east into China, and west into Europe via trade routes, said Black.

Even if the second pandemic had come from China, the Zheng He theory isn't feasible -- as Black points out, if Zheng He's ship was carrying plague-infested rats, the whole crew would most likely be dead before they reached Africa.

China's brush with modern plague

But when it comes to the third pandemic, there are fewer questions. This time, scientists are sure it originated in China in the 19th century , in what is now the southwestern province of Yunnan

That bubonic plague outbreak made its way to Hong Kong -- then a British colony -- and from there, spread via trade routes to other parts of Asia and the United States.

"It's undeniable that there was this pathway of transmission from China to the outside world," said Jack Greatrex, who is working on a PhD at Hong Kong University about the history of the plague in Hong Kong.

Plague inspectors on a street of Hong Kong, around 1890.

That outbreak sparked the third global plague pandemic. But it was another plague outbreak that would help shape China's future.

In the 1910s, there was another outbreak of plague in Manchuria -- now northeast China. Thousands were killed by pneumonic plague, the most severe strand.

At the time, parts of China were occupied by foreign powers. Both the Russian and Japanese empires claimed they could manage the plague in Manchuria better than China, which showed China that disease could be a "security disaster" as it "legitimated colonial meddling," said Miriam Gross, who studies public health in China and is a professor at the University of Oklahoma.

When the founder of modern China, Mao Zedong, came to power in 1949, he made disease control a priority. There were a number of reasons for that, but one was to show that China could handle its own affairs and didn't need outside help, Gross said.

Chinese Cultural revolution poster about the so-called four pests: mosquitoes, rats, flies and sparrows.

So Mao put in place a number of measures to control the country's rampant disease. One of his most famous and unusual proposals was the "Four Pests Campaign," where Mao called for rats, flies, mosquitoes and sparrows to be eliminated. The rats were to be killed to control schistosomiasis, which is sometimes translated in English as "plague" although it is a different disease.

But the Four Pests Campaign led to the slaughter of millions of wildlife, which disrupted the country's ecology and contributed to a mass famine during which millions of people died.

Ultimately, though, China did improve its overall health care across the country. Nevertheless, the plague -- which had not been the main focus of the health push -- has occasionally reared its ugly head . Yunnan was hit by another breakout between 1986 and 2005 , and another case was diagnosed in Yunnan in 2016.

Why we're so fascinated by the plague

Centuries on from the Black Death, people around the world continue to be transfixed by the plague in a way they're not by other diseases.

These days, the plague is hardly the biggest health risk facing many countries. In 2017 alone, 219 million people caught malaria and 435,000 people died of the disease. By contrast, between 2010 and 2015, 584 people died of the plague worldwide, according to the World Health Organization.

, state media China Daily While the plague can be deadly if untreated, patients can easily be treated with antibiotics. After the plague diagnosis in China, the Chinese Center for Disease Control and Prevention said there was an extremely low risk of it spreadingstate media China Daily reported

But even if the disease isn't a major threat for most countries, it still interests scientists and historians, who are continuing to make discoveries about the Black Death, despite it occurring hundreds of years ago.

Staff members from a local disease control center wear prevention clothes and masks before entering a plague surveillance lab in Sichuan Province of China in August 28, 2019.

Greatrex, from Hong Kong University, said the plague continued to be haunted by its history. "You hear of the plague, and instantly you think of Black Death which ravages Europe, it has that enormous historical baggage," he said. "It's where lots of our ideas about what it means to have an epidemic comes from."

Black, the historian, said the fascination with the Black Death comes from a deep cultural memory in the Middle East and Europe, where the disease was written about for centuries.

However, he said other diseases -- such as malaria and Ebola -- should be of greater concern.

"It's so central to Western identity," he said. "It's part of our past, where something like malaria, which is so much more devastating in the last century, it doesn't interest us."