How Plague Works

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The History of Plague

Today, some of the illnesses that cause the most alarm are newly discovered, deadly diseases. 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 [source: WHO]. A virus also causes Ebola, identified in 1976. Ebola has a mortality rate of up to 80 percent [source: CDC]. The first known case of HIV was reported in the 1950s. Scientists isolated the virus responsible in the 1980s [source: Aegis].

People have reacted to the appearance 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 A.D. It claimed about 100 million victims in Europe, Asia and Africa.
  • The Black Death moved across Europe in the 1300s. About a third of Europe's population died. There were about 50 million total 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.