New Wondrium Black Death Series Says Plague Preferred Summer

cold winter months hindered spread of infection

By Jonny Lupsha, Wondrium Staff Writer

Unlike flu and COVID-19, the Black Death spread the fastest in summer. The Black Death was a bacteria and relied on warm weather factors to proliferate. This week on Wondrium Shorts, uncover why summer was plague season.

Influenza and COVID-19 are two of the most well-known viruses on Earth. In the cold winter months, they spread like wildfire, since viruses thrive in cold, dry air. We’re also more likely to be indoors close together, making transmission by coughing and sneezing skyrocket. The Black Death, on the other hand, is a bacteria and needs to be put directly into the body, by methods such as flea bites. During the winter, there are fewer fleas hopping around biting animals and humans.

This is only one example of why plague prefers summer. In her video series The Black Death: New Lessons from Recent Research, Dr. Dorsey Armstrong, Professor of English and Medieval Literature at Purdue University, lays out the reasons fewer victims contracted The Black Death in cold weather.

The Family and Friends Plan

Viruses are much smaller and lighter than bacteria, and travel through the air more easily and in smaller droplets. This is why health organizations recommend six feet of social distancing. Bacteria like the Black Death can only travel about a foot when an infected person coughs or sneezes.

Any parent knows that children will cough or sneeze directly onto them—or, in a close-knit household, illnesses circulate among family members.

“While plague is rare in winter, it is still possible that an infected flea might bite a human being during this time; the infected person would become ill with the pneumonic form of the disease, and then that person might spread the disease to the rest of the family by coughing or sneezing directly onto them, or they might do so within a foot of distance,” Dr. Armstrong said.

This is also why plague was less likely to spread outside the household in the winter. Visitors are far less likely to come visit a sick household, let alone brave cold winter weather, and much less get within a foot of a sick person.

Fleas and Rats

“Plague tends to disappear in winter because there are simply fewer flea bites,” Dr. Armstrong said. “When it’s cold, the rate at which fleas lay new eggs declines. At the same time, those eggs that are laid are less likely to complete gestation, so in winter the flea population tends to decrease dramatically.”

Additionally, according to Dr. Armstrong, the blood of plague-infected rats is less lethal below 50 degrees Fahrenheit. In order to understand why, it’s important to understand how plague is contracted. The plague bacterium infects fleas by blocking the entrance to the flea’s digestive system while the flea feeds, so the flea fills its proventriculus—a sort of “pre-stomach” stomach—with blood until it has to throw it back up into its host.

“But while fleas may be sucking away on plague-infected rats just as merrily in winter as they are in summer, the amount of plague in that rat blood in cooler temperatures is less than in warmer temperatures,” she said. “This means that the blockage of the proventriculus of the flea will take longer to happen, as some nourishment is still getting through to the flea’s main digestive system in winter.

“And with the increased die-off of fleas and flea larvae in the winter, that means fewer opportunities for them to infect humans.”

This article is part of our “Deeper Dive” series where we examine the stories behind our Wondrium Shorts on YouTube.

Edited by Angela Shoemaker, Wondrium Daily