Why does plague keep plaguing humans?

Public health officials in Colorado recently confirmed that a man has plague.

Throughout human history, different infectious diseases have taken the mantle of “most deadly disease” infecting humans. In the past century alone, the human population has experienced many pandemics: COVID, HIV and various influenza outbreaks – to name a few. Some have lasted for centuries and persist today, such as tuberculosis. Others are often thought of as being consigned to the history books.

Before the 20th century, the most-deadly-disease mantle was held in Europe and surrounding areas by the bubonic plague. Three major pandemics of this disease have occurred in the past 1,500 years. The first occurred from the fifth to the seventh century, killing about 15 million people in the Mediterranean basin, and heavily affecting the Byzantine, Sasanian and Roman empires.

A much larger second outbreak, called the Black Death, then occurred in 14th-century Europe, where over 50 million people, around 50% of the entire European population, died from this disease.

The third wave of this pandemic then occurred globally in the 19th and 20th centuries, killing a further 30 million people worldwide, many of these in China and India.

However, from the 1960s onwards, cases dropped dramatically, and the bubonic plague is not often considered a modern disease. Despite this, a new case was recently reported in the US, renewing interest in this disease.

Although no longer common in many parts of the world, the bubonic plague still exists in geographic pockets and can spread in communities if the right mix of conditions are present.

The bubonic plague, or plague for short, is caused by a bacterium called Yersinia pestis. There are three types of plague caused by this pathogen, each with a different part of the body as the main site of infection: pneumonic is mainly lung-based, septicemic is mainly blood-based, and bubonic is mostly in the lymph nodes.

Although one form can turn into another during an infection, generally which form a person has is driven by how they were infected.

Bubonic plague is the form of Y pestis infection that is spread by fleas that live on small animals, mostly rodents such as the house and field rat. These rodents serve as reservoirs for the bacteria: they show little-to-no symptoms but can pass the bacteria to others, including humans.

This transmission from rodents to humans takes place via fleas. These insects bite the rats and afterwards may jump and bite a human, injecting the plague bacterium into the lymphatic system of the human. The bacteria then travel through this system to the lymph nodes and infection begins.

The main symptom of bubonic plague is swollen lymph nodes, usually in the neck, groin, thighs and armpits. These swollen nodes, called buboes, can cause the tissue around them to turn black and die. They may also burst open, releasing the pus inside.

Other symptoms include fever, headaches and vomiting, and the pathogen may spread to other parts of the body, such as the lungs and blood, causing other forms of the plague. Bubonic plague kills 30-60% of people, whereas pneumonic and septicaemic are always fatal if left untreated.

So why was this so prominent hundreds of years ago but barely heard of today? It is all about having that crucial combination of vector (flea), reservoir (rodent) and bacteria (Y pestis) all occurring together and in close contact with humans.

Before the 19th century, people primarily thought that disease was spread by miasmas: noxious forms of air. It was only after the 1880s that people realised that microscopic organisms transmitted between humans, animals and the environment can cause diseases.

Under control

From this, sanitation improved in many parts of the world, separating rodents from humans and breaking the cycle of plague transmission. The invention of antibiotics, especially fluoroquinolones from the 1960s onwards, further drove down the cases of plague as proper treatment could now be given for all forms.

Today, we still see cases of plague in specific hotspots, mainly in Asia, Africa and South America. The Democratic Republic of Congo, Peru and Madagascar are the countries with the most cases.

Madagascar alone has dozens of cases a year, with more major outbreaks occurring in 2014 and 2017 (the latter had over 2,000 cases). The dense forest areas are home to many rodents, and contact between people and these ecosystems is the cause of these modern outbreaks.

The plague will probably never be eradicated. Because of its complex transmission network of fleas, rodents and humans, it is nearly impossible to find, control and treat all these aspects. However, through proper handling of animals, separation of natural reservoirs and humans, and quick and effective treatment, the number of plague cases is decreasing every year, with hopes of negligible case numbers in sight.

Conor Meehan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.