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How Plague Works

The History of Plague

Workers transport dead bodies during the Great Plague of London
Workers transport dead bodies during the Great Plague of London. The Great Plague, lasting from 1665 to 1666, was the last major epidemic of the bubonic plague to occur in England duncan1890/Getty Images

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Today, some of the illnesses that cause the most alarm are newly discovered, deadly diseases, usually spread by viruses. Scientists isolated avian flu H5N1 in 1996. Person-to-person spread is rare, but the virus has a mortality rate of about 60 percent in humans. A virus also causes Ebola, identified in 1976. Ebola has an average mortality rate of 50 percent. The first known case of HIV was reported in the 1950s. Scientists isolated the virus responsible in the 1980s. The mortality rates for this disease dropped from 16.3 per 100,000 in 1998 to just 3.7 per 100,000 in the U.S. in 2017, thanks to the availability of drugs to treat it.

In 202, the novel coronavirus (COVID-19) outbreak quickly caused a worldwide pandemic, shuttering schools, churches, businesses and society in general in an effort to limit spread of the damaging respiratory illness. The severe social distancing was forced due to the fact that the airborne COVID-19 is exceptionally contagious, and also because many people don't show symptoms at all, so they can easily and unknowingly transmit the disease to others.

People have reacted to the appearance of each of these diseases with fear and dread. A major outbreak of plague today would spark a similar reaction. But unlike many of today's newsmakers, plague comes from an old bacterium rather than a new virus. Researchers believe that Yersinia pestis diverged from the less-lethal Yersinia pseudotuberculosis about 20,000 years ago [source: Huang]. Some believe that plague lived in rats before humans existed. Descriptions of a disease resembling plague also appear in ancient texts, including the Christian Bible.

On top of being old, plague is virulent or highly infective. It generally gets the credit for three major pandemics, or massively widespread epidemics:

  • Justinian's Plague lasted from 542-546. It claimed about 100 million victims in Europe, Asia and Africa.
  • The Black Death moved across Europe in the 1300s, killing about a third of the continent's population. In total, there were about 50 million deaths in Europe, Asia and Africa.
  • The Third Pandemic started in Canton and Hong Kong during the late 1800s. Ships carried the illness to five continents. Thirteen million people died in India alone [source: WHO].

An infamous epidemic, the Great Plague of London, took place during the 16th century. The Great Plague killed up to a fifth of London's population, but the disease did not spread around the world. In other words, it didn't escalate from an epidemic to a pandemic.

During each of these epidemics, no one knew what caused the disease or how it spread. During the Black Death, for example, many blamed the illness on toxic miasmas, so people focused on keeping bad air away. Plague doctors, who usually had little to no medical training, wore masks stuffed with herbs to filter the air. In some cities, people blamed dogs and cats for the illness. The resulting slaughter of rats' natural predators may have encouraged the spread of the disease. In Rome, on the other hand, a large population of feral cats may have provided people with additional protection. A study of tree rings released in 2015 also suggests that, before the disease spread to Europe, the initial Asian reservoir for plague-carrying fleas may have been gerbils rather than rats.

Historical records describe a number of different symptoms during these and other outbreaks. These include rashes, nausea, sensitivity to light, diarrhea and coughing. Swollen, painful buboes appear consistently in most accounts. This is one of the reasons why plague takes the blame for so many pandemics.

The idea that bubonic plague was behind these pandemics has become part of the conventional wisdom – it's something everyone knows. However, some researchers have doubts. Next, we'll take a look at the bacterium behind plague and why some scientists believe it didn't cause the Black Death.

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