Plague is still deadly, but another Black Death is highly unlikely

As one of humanity's oldest diseases is in the news due to a death in Siberia, RCSI's Steve Kerrigan analyses if plague could once again become a major global threat.

The recent death of a 28-year-old laboratory worker at an anti-plague research institute in Siberia has put one of humanity's oldest diseases back in the headlines. Darya Shipilova died after developing severe pneumonia. Because she worked at the Irkutsk Anti-Plague Research Institute, her death prompted concern that she may have been infected with plague, but this has not been confirmed.

Russian authorities say no plague cases have been detected among her contacts, while the World Health Organization has asked for further information. For now, the wider public-health risk is considered low, but the episode has prompted an obvious question: if the bacterium that caused the Black Death is still with us, could plague once again become a major global threat?

Plague is caused by Yersinia pestis, a bacterium that circulates naturally among small mammals and the fleas that feed on them. It was responsible for some of the most devastating pandemics in history, including the Black Death of the 14th century, which killed tens of millions of people across Europe and beyond.

But plague never disappeared after that pandemic and human cases still occur, particularly in parts of Africa, Asia and the Americas. The WHO says most reported cases between 2019 and 2025 occurred in the Democratic Republic of the Congo and Madagascar.

Humans are most commonly infected through the bite of an infected flea. The bacteria travel through the lymphatic system to nearby lymph nodes, where they multiply and cause the painful swellings known as buboes.

This is bubonic plague, the most common form. If the bacteria enter the bloodstream, septicaemic plague can develop. If they infect the lungs, the result is pneumonic plague. The latter is especially important because it can spread from person to person through respiratory particles, in the same way we transmit viral respiratory diseases like the common cold.

How pneumonic plague kills

What makes Yersinia pestis particularly dangerous is its ability to interfere with our immune response. The bacterium has a structure known as a type III secretion system, which works rather like a microscopic syringe. It injects proteins called Yops directly into immune cells, disrupting the processes those cells use to respond when they are infected. This gives the bacteria something extremely valuable: time to multiply before the body mounts its full response.

This can produce a striking two-stage infection in pneumonic plague. Early on, bacteria multiply rapidly in the lungs while the inflammatory response remains surprisingly subdued. This can then switch to intense inflammation, severe pneumonia and rapid deterioration. Untreated pneumonic plague can be fatal extremely quickly. The WHO warns that without prompt treatment it can kill within around 18 to 24 hours after symptoms begin.

Severe plague can also spread through the bloodstream, damaging tissues and organs and disrupting the body’s circulation and clotting system. Bleeding beneath the skin, tissue damage and gangrene can occur in advanced disease.

Deadly does not mean highly transmissible

So, if plague is this dangerous, why is it not another COVID-19? How deadly an infection is and how easily it spreads through a population are two different things. COVID-19 was extraordinarily difficult to contain because SARS-CoV-2 spread efficiently through the respiratory route, including from people who had no or few symptoms or had not yet realised they were infected. By the time one person became seriously ill, many other infections could already have occurred.

Plague behaves very differently. The commonest form, bubonic plague, is not normally transmitted directly between people. Person-to-person transmission is primarily a concern with pneumonic plague and generally requires relatively close exposure to an infected person.

That makes outbreaks much easier to target. A suspected pneumonic plague patient can be isolated. Close contacts can be identified and monitored. Crucially, people who have had significant exposure can be given antibiotics before they become ill.

Antibiotics are perhaps the biggest reason plague represents a very different threat today. Yersinia pestis remains extremely dangerous if untreated, but it is a bacterium and can be killed by antibiotics. Early diagnosis and treatment dramatically improve the chances of survival.

Could the Black Death happen again?

Yersinia pestis is still with us and, because it circulates naturally in wild animals, it is unlikely ever to disappear completely. Sporadic human infections will continue to occur.

But a modern case of plague is not the beginning of another Black Death. We understand what causes it and how it spreads, we can identify it quickly, treat it with antibiotics and protect people who have been exposed.

Plague can still be extraordinarily dangerous if it is missed or treatment is delayed. But the world in which this ancient bacterium circulates has changed enormously. The bacterium has changed surprisingly little, but we have.

This was originally published on originally published on RTÉ Brainstorm. Read the original article here.