Reflections on Reading Medical Materials and Historical Documents About Plague

Recently I have read a large amount of medical material and historical documents concerning plague. Plague may be the infectious disease with the greatest impact on humanity in history; it has claimed countless lives and caused several population collapses in history-on many occasions killing more than half the population where it erupted.

In 2019, plague also appeared in Inner Mongolia, though on a very small scale, with only a few scattered cases in a few places of the Inner Mongolia Autonomous Region. The grasslands of northern China are a natural focus of plague, so a few scattered cases are not surprising. But these cases were unconnected to one another, so this is actually a signal worth being alert to.

Historically, plague once swept across Eurasia. Europe's Black Death killed about a third of Europe's population; the Black Death was plague transmitted by black rats. To this day, the Yersinia pestis that causes plague has not vanished from the earth. Rodent burrows are a hotbed for Yersinia pestis, which spreads through rodents and the fleas that parasitize them.

Humankind's war against plague may span more than two thousand years, and recorded plague epidemics are innumerable. The Mongol hordes once swept across Eurasia, yet in the end the Mongols retreated step by step. Historians who study plagues believe that what truly defeated the Mongols was plague. It was precisely because vast numbers of campaigning soldiers were infected with plague-and the mortality rate was very high in that era-that Mongol military strength was greatly weakened.

During the Jin-Yuan period, plague raged across China, killing more than half the residents of some regions. Plague not only caused massive casualties but also had a huge impact on economic and social development. The great plague epidemics that broke out in China during the Jin-Yuan period and Europe's Black Death both brought economies to a standstill. After Europe's Black Death, the Renaissance and even great revolutions erupted, ending Europe's dark Middle Ages.

So this tiny microorganism is the greatest threat to us human beings.

In the struggle against plague, humanity has accumulated rich experience. Some of this experience has become taboos among the people in regions where plague once erupted, and these taboos have effectively protected lives. But outside invaders unfamiliar with local customs know nothing of these taboos, and they trigger plague by violating them.

The Jin-Yuan plague is said to have originated in Yunnan. The local Yunnan people, who had suffered plague, had a taboo: they would move out of any house where rats had died and find another dwelling. Although many locals did not know the purpose of this custom passed down from their ancestors, they still followed it. The result was that although Yunnan is also a natural focus of plague, it long avoided outbreaks.

When the northern nomads invaded Yunnan, these cavalrymen knew nothing of the local taboos. The result was that they ignited a plague epidemic and, through the movement of armies, carried plague to all parts of the world.

The 1910 plague outbreak in Northeast China was likewise related to the "rush to Guandong" of that era. Vast numbers of disaster refugees from inside the pass rushed into the Northeast, and they too knew nothing of some of the region's folk taboos.

The Northeast, because earlier nomadic campaigns had brought back plague, had Yersinia pestis lingering on the rodents of the grasslands, making it a natural focus. After suffering plague, those grassland tribes formed their own taboos: when hunting marmots, they killed them only with arrows and did not use cages to catch them, and they did not hunt lethargic marmots. These customs effectively spared them direct contact with sick rats.

But the Shandong people who rushed to Guandong did not know this taboo; many of the Shandong migrants of that era worked in the fur trade. Marmot pelts were highly sought after, and these people used every means to catch marmots, igniting the plague outbreak. In the area beyond the pass, more than five thousand people died of plague every day.

In the past, plague spread mainly through merchants and through armies in wartime. In an age when transport was underdeveloped, plague did not spread fast, and natural barriers-a great river, for instance-could halt an epidemic. But times have changed: if plague erupted in a modern city, it could spread around the globe within thirty-six hours. Once a city's population exceeds 500,000, microorganisms such as bacteria and viruses are enough to ignite a plague there and spread it rapidly elsewhere.

How did the ancients overcome plague epidemics? Strictly speaking, most of the time it was through "natural immunity." As with Europe's Black Death, it ultimately reduced Europe's population: all who could be infected were infected, all who could fall ill fell ill, and all who could die died; only when the survivors carried antibodies did the epidemic subside.

Early people discovered the routes of plague transmission, so the ancients had their own quarantine measures. During outbreaks, all merchant ships were barred from landing in various ports; they had to stay at anchor for forty days before crews were allowed ashore. This method of quarantine had some effect.

More effective, however, was case detection and isolation. Camus's novel The Plague vividly describes the real scene of a whole city locked down after a plague outbreak. I believe people now confined at home will read that novel with a sense of deja vu.

Today, looking at the taboos of regions that once suffered plague and at the ancients' quarantine and anti-epidemic methods, one feels a great deal. Humankind has never had it easy living on this earth. Plague and war are humanity's greatest enemies; judging from the historical data, plague has caused far more deaths than war. Historically, plague effectively checked excessive population growth and protected the earth's climate and environment.

In modern times, however, great plagues that infect most of the globe have been rare; besides the Spanish flu of a hundred years ago, which had such enormous power, only today's COVID-19.

Usually, once the pathogen of a great plague has spread widely among people, it never easily vanishes from the earth again, and the antibodies humans develop against a pathogen are generally effective for only 30-50 years. It takes humanity about 120 years to adjust to and coexist with a new pathogen. That is to say, once a great plague like the plague occurs, several generations may have to live under its shadow; it is by no means something that ends in a year or two.

Yersinia pestis is a zoonotic pathogen; humans and rodents are its main hosts, and fleas can also transmit it. So in the present day, plague is classified as a Class A infectious disease and is strictly guarded against in every country.

Strictly speaking, humankind is incapable of eradicating any infectious disease. To this day, the only infectious disease humans have completely eradicated may be smallpox; the other once-widespread infectious diseases still threaten human health-though this claim is disputed, and some believe the smallpox virus still exists in nature and has not been eradicated.

Microorganisms repeatedly teach humanity lessons, and humanity keeps trying to evade them. Many of humankind's seemingly puzzling customs may be scars left by historical plagues. Some scholars even hold that the rise of religion is closely related to plague. Because plague killed so many loved ones, people needed some ritual to soothe their souls, and so religion arose.

Since the establishment of modern public-health systems, preventing infectious diseases has become more scientific and clear-cut, and humanity has worked to keep its distance from wild animals. But this also requires everyone to abide by the rules; as long as a few people among humanity refuse to comply, every one of us is in great danger. It is like the bucket theory: humanity's overall safety depends on the most dangerous element, not on ordinary people.

Whether COVID-19 will torment humanity for a long time to come, as plague has, we cannot yet conclude. Some studies have already found that the novel coronavirus exhibits zoonotic transmission; even cats and dogs, domesticated long ago, can be infected with it.

The latest reports show that even tigers in zoos have caught COVID-19 and developed symptoms. Will other mammals be infected too? Will insects such as fleas and mosquitoes transmit the novel coronavirus? Are there other routes of transmission? These require further research to know.

We should not treat these unknowns lightly, nor should we casually dismiss them to soothe people's feelings; we should study them carefully and guard against them cautiously. This is the responsibility of us medical people.

Once Pandora's box has been opened, it will not be simple for us humans to rid ourselves of COVID-19. This highly transmissible, high-mortality virus is making human survival harder than before.

Returning to the way things were will be no easy matter.