Most students learn about plague in history

Most students learn about plague in history class due its huge impact on human history. In this, the first of two lessons on plague, we will look at the disease from a microbiological perspective, focusing on the causative agent: Yersinia pestis.

Biological Weapon of Choice in the 1300s

Imagine you were born in the early 1300s in the Black Sea port city of Caffa, part of today’s Ukraine. Usually, Caffa is a bustling port, full of people and goods from the far reaches of Europe and Asia. But today has an ominous air. A group of Italian merchants has unwisely crossed the Mongols, who have controlled the area for generations.

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You know that nothing good can come of this. The merchants feel safe, holed up within the walls of Caffa. Then, you hear the screams coming from inside the city, and you start to smell the stench.Suddenly, something large flies over the wall, landing a few feet from where you’re standing. Now the stench is unbearable.

You look closer at the projectile and recoil in horror. It’s a dead human body. The Mongols are catapulting dead humans over the walls of Caffa! In order to drive out the merchants, the Mongols have started raining infectious death over the entire city.This is one of the earliest confirmed cases of a biological agent used as a weapon. Each of those bodies carries billions of tiny bacteria capable of incredible amounts of destruction.

The Mongol weapon of choice is Yersinia pestis, the bacterium that causes plague.

Yersinia pestis and the Rat Flea

Yersinia pestis is a small, Gram-negative, rod-shaped bacteria that causes plague. ‘Gram-negative’ refers to Yersinia having a cell wall with a thin peptidoglycan layer and an outer membrane, making these cells appear red in the classic Gram stain.

Interestingly, with a careful staining procedure, Yersinia cells stain very heavily on each end and take up very little stain at the center, giving the cells a unique ‘safety pin’ appearance.Yersinia pestis is a natural pathogen of rats. Most rats that carry the bacteria develop plague and die very quickly. There are a small percentage of rats, though, that are immune to plague. These rats become chronic carriers, capable of spreading the disease to many other rats, rodents, mammals, and even humans. But, it is not the rat you have to fear.

It is actually another parasite living on the rat that transmits plague: the rat flea.If rats are considered the natural host of Yersinia pestis, rat fleas are considered a plague vector. A vector is an organism that transmits a disease from an infected host to a susceptible host. When a rat flea bites an infected rat, it ingests the Yersinia bacteria in the rat’s blood. Once in the flea’s stomach, Yersinia has a fascinating mechanism for spreading itself to other hosts. The bacteria multiply, making millions of copies of itself, which eventually blocks the flea’s digestive system.

Unable to swallow the blood it feeds on, the increasingly hungry flea becomes a ravenous biter. It attempts to feed on any warm-blooded animal it can reach, switching hosts frequently as it searches for a blood meal it can actually swallow. Every time it bites, it swallows some blood, but since the flea’s stomach is so full of bacteria, it vomits up the blood along with some Yersinia cells. The cells get injected right into the new host. Every host is left with an itchy, red bite and a plague-inducing inoculation of Yersinia pestis.


Plague‘ is the name given to any disease caused by the bacterium Yersinia pestis. Plague is considered one of the few history-changing diseases, killing hundreds of millions of people in the last 1,500 years.

The first major pandemic occurred in the 6th century AD. It began in Africa and spread to the entire Mediterranean basin. Estimates of this pandemic alone put the death toll at about 100 million, helping to contribute to the fall of the Roman Empire. In the 1300s, the more infamous ‘Great Plague,’ also known as the ‘Black Death’ pandemic, started in China, eventually reaching Europe. Over the next several hundred years, plague decimated the population of Europe, killing between 25-33% of the population of the continent with every new outbreak.Some outbreaks claimed upwards of 60 million lives, leaving too few healthy laborers to even bury the dead, let alone providing for daily needs of the survivors.

The last great pandemic started in the late 1800s, spreading from Asia in rat-infested ships. By 1900, some of those rats had reached San Francisco, depositing Yersinia pestis in the New World.Once in the United States, Yersinia pestis found a new host: the prairie dog. These rodents, in addition to chipmunks, squirrels, and the usual rats and mice, have maintained plague in the United States to this day. Fortunately, not many people are ever directly in contact with these rodents, so actual cases of plague are rare.

The U.S. averages around 10 cases of plague every year.

In developed countries, plague is a rarity, but cases have been rising recently in less-developed countries. Overcrowding and poor sanitation has led to an increase in rat populations. More rats means more fleas, which means more vectors for Yersinia infection.

Types of Plague

There are three major types of plague.

All are caused by Yersinia pestis; only the route of infection is different. Bubonic plague is the classic illness caught through the bite of an infected flea. This type represents the majority of cases, with the bacteria invading the lymph nodes of the patient.Pneumonic plague is a highly contagious form of plague passed from person to person through inhaled aerosolized respiratory droplets. Septicemic plague is plague in which the bacteria have spread to the bloodstream of the host. This infection can spread to the rest of the body, and untreated cases are nearly 100% fatal.

We will cover the symptoms, diagnosis, and treatment of these three forms of plague in another lesson.


Prevention of plague involves two pathways: limit exposure to those infected or eliminate the vector. People afflicted with plague should be quarantined to prevent further spreading.

Eliminating the flea vector usually focuses on eliminating the rodent carriers. This can be as simple as cleaning up and eliminating rat-friendly areas or as intensive as trapping rodents and poisoning burrows. Currently, there is no available vaccine in the United States.

Several are in development, but none are close to being released.

Lesson Summary

Let’s review.Yersinia pestis is a small, Gram-negative, rod-shaped bacteria that causes plague. Plague is a severe disease that has killed countless millions of people while changing the course of history throughout the past 1,500 years. Yersinia pestis is a natural pathogen of rats. The bacteria are passed from rat to rat or rat to human primarily through the bite of an infected flea vector.

A vector is an organism that transmits a disease from an infected host to a susceptible host.The United States has about 10 cases of plague every year, making it a rarity here. These cases are usually associated with contact with prairie dogs, chipmunks, or squirrels, the primary Yersinia carriers in the U.S.

Plague is still a serious health threat in less-developed counties.There are three main types of plague. Bubonic plague infects the lymph nodes and is caught through flea bites. Pneumonic plague infects the lungs and is caught by inhaling aerosolized respiratory droplets. Septicemic plague infects the blood and occurs when pneumonic or bubonic plague spreads to the bloodstream.Quarantining infected individuals and reducing rat and flea exposure are the two major tactics for preventing plague.

Learning Outcomes

Once you’ve completed this lesson, you’ll be able to:

  • Summarize historical outbreaks of plague
  • Describe the structure and transmission of Yersinia pestis
  • Explain how the three main types of plague are transmitted and what parts of the body they affect
  • Identify methods of preventing plague

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