Plague comes from the ancient Greek word for misfortune or blow. Bacteria named Yersinia pestis are the cause of plague. They get into humans and other animals through open wounds, the air, or flea bites.
A smear of bubonic plague, Yersinia pestis
Scanning electron micrograph of Yersinia pestis
Credit: Justin Eddy, Lindsay Gielda, et al, CC BY-ND
In humans, Yersinia pestis travel around our bodies and multiply. When making their home in our lymph nodes they become bubonic plague. Once there, they cause painful swelling, fever, chills, headache, diarrhea, constipation, rapid heartbeat, anxiety, staggering gate, and death if untreated. Sometimes the bacteria prefer to move into our lungs and develop into a pneumonic plague that can spread through our coughs. Bubonic and pneumonic plagues can lead to a blood-born, septicemic plague that causes nerve and brain damage—killing almost everyone who gets it.
A flea infected with Yersinia pestis
Credit: Smith Collection / Gado / Getty Images
Engraving of a flea
Plague needs fleas and rodents to infect humans. Yersinia pestis started travelling through flea bites thousands of years ago. Fleas feeding on infected animals pick up the bacteria and begin feeling hungry and sick. The fleas search for more food, consuming anything’s blood, and vomiting bacteria into their fresh wounds.
A black rat on a rope
The rat is a flea’s perfect vehicle. They follow humans around the world, adapt to our homes and behaviors, and bear many young. They are mobile flea restaurants, moving and feeding fleas across continents, bringing them closer to humans.
Map of Mongolian Empire at its height (c. 1259-14th century) with contemporary national borders
Credit: Logan Museum of Anthropology
From 700-1250 CE, European summers were long and the winters were short. This was a prosperous time, until the lands became exhausted and the winters grew longer as the 14th century approached. Millions were starving to death and growing weak.
In Asia, the Mongol Empire was expanding and creating trade networks for goods, services, military, and culture. These networks helped rats, fleas, and bacteria cross the continent and move into Europe.
Climate change and trade networks made it possible for the bubonic plague, known as the Black Death, to spread and take the lives of tens of millions of people in Europe, Asia, Africa, and the Middle East—killing about 25% of the world’s population.
The plague is not a cunning mastermind, but an opportunist that takes advantage of climate, environmental, socio-political, and economic changes.
After the Black Death, survivors were resistant to the disease. Their resistance would help protect their descendants in future outbreaks.
In Europe, the decline in population led to more food and land for survivors. The survivors were eating better, living longer, and gaining opportunities for social mobility, allowing for new political systems to develop.
Saint Sebastian Interceding for the Plague Stricken
Oil on Wood
France, 1497-1499 (Renaissance)
Credit: The Walters Art Museum
A rat running across a street in Manhattan
Credit: REUTERS / Carlo Allegri
There is no difference between modern plague outbreaks and their historic relatives, except for the use antibiotics, which reduce the threat of deadly outbreaks. Despite this, about 1,000 people die of plague each year, and the use of antibiotics may lead to drug-resistant strains of Yersinia pestis.
Unstable weather conditions associated with climate change and human development are leading to changes in rodent habitats. This may cause explosions of rodent populations in cities, which increases the likelihood of plague outbreaks.