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What Causes the Plague?

Flea bites or aerosolized respiratory droplets can spread disease

Aug 6, 2009 Kenneth Rosen

One species of bacterium can cause two different types of plague, the bubonic plague or the pneumonic plague. Which one occurs depends on the manner of transmission.

A recent outbreak of plague in an isolated province in China has again raised interest in this deadly disease. In the 1300s in Europe the “Black Death” decimated a large percentage of the population. The plague was a result of infection with the bacterium known as Yersinia pestis. Spread through the bites of fleas carrying the bacterium from rats to humans, the infected person developed what are known as “bubos”, red-black swellings which erupt from heavily infected lymph nodes. This is how it came to be known as the “bubonic plague”. This is not the form of the disease currently being seen in China; there it is the “pneumonic plague”.

Infectious disease has probably done as much or more than armed conflicts over the course of history to change human behavior and to ravage human populations. In the middle ages, as the Black Death swept across Europe, villages would try to stop visits from outsiders, in effect trying to isolate themselves from possible carriers of the plague. It had little effect. It is estimated that more than one quarter of the European population perished as a result of the plague in the 1300s.

The Difference Between Pneumonic and Bubonic Plague

Both pneumonic plague and bubonic plague are caused by the same bacterium, Yersinia pestis. The form of the disease that develops is dependent upon how the person becomes infected. Bubonic plague results when a person is bitten by a flea carrying the bacterium and it infects the blood stream. Pneumonic plague occurs when someone breathes in aerosolized droplets from the cough of someone infected with the bacterium in their lungs and upper airways.

Without rapid intervention with antibiotics, something that was obviously unavailable during the Middle Ages, mortality from infection can be both rapid and extensive. For bubonic plague, it can take 4 to 7 days for the disease to develop after a flea bite. In the case of pneumonic plague, the onset of disease is much more rapid, in as little as 24 hours. Without medical intervention, death rates from bubonic plague can be as high as 75%, while pneumonic plague can produce 90% mortality.

Natural Reservoirs for Plague and its Transmission

The plague bacterium can occur in numerous types of rodents from rats to prairie dogs to squirrels. When an infected rodent is bitten by a flea, the flea can consume the plague bacterium in its blood meal. The bacterium can live in the gut of the flea and is transmitted when the flea bites a new victim. The plague bacterium grows differently at different temperatures. When it is in the flea, the insect’s normal temperature is low enough that the bacterium does not express certain characteristics. When transmitted to a warm-blooded animal, the bacterium expresses new proteins that help it to fight off the defensive systems of the body.

Plague can be treated with antibiotics if detected early enough. Modern medical treatments make the outbreaks that altered the landscape of medieval human society extremely unlikely. TO read more about plague and the bacterium that causes it, visit the US Center for Disease Control webpages.

The copyright of the article What Causes the Plague? in General Medicine is owned by Kenneth Rosen. Permission to republish What Causes the Plague? in print or online must be granted by the author in writing.

Fleas can Spread Plague