It is among the deadliest diseases in the world, capable of killing its victims within 24 hours if left untreated.
Some historians and researchers believe pneumonic plague drove the mass transmission of the Black Death, which wiped out half of Europe in the 14th century.
And this week, it’s been thrust into the spotlight.
The disease is believed to have killed 28-year-old lab worker Darya Shipilova in Siberia on Friday – days after she allegedly told staff she had accidentally broken a test tube containing live plague bacteria, Yersinia pestis.
In a race to protect against a potentially catastrophic outbreak, Russian officials – who maintain that the researcher died from ‘unidentified pneumonia‘ – have quarantined and begun testing around 200 people that Ms Shipilova had been in contact with.
The case has sparked global fears, given the disease is highly contagious and the deadliest form of plague there is – killing up to 100 per cent of its victims.
But how does it have such devastating consequences so rapidly? And what exactly does it do to the body?
Here we explain, step by agonising step, how pneumonic plague can kill.
Pnuemonic plague is believed to have killed 28-year-old lab worker Darya Shipilova on Friday
Contracting the disease
The first thing to note is the difference between pneumonic plague and the two other main forms of the disease: bubonic and septicaemic plague.
Bubonic is the most common form, generally transmitted after being bitten by an infected flea or animal, while septicaemic plague occurs when the infection spreads into the bloodstream – often as a result of untreated bubonic plague worsening.
Pneumonic plague, on the other hand, mainly spreads through small droplets released from the mouth or nose when an infected person or animal breathes, coughs or sneezes.
According to the Centers for Disease Control and Prevention, it is the only form of plague that can be spread between people.
Once exposed, the bacteria can begin to incubate inside the body, and within as little as an hour, begin to live inside pockets of air in the lungs.
Incubation
Once the bacteria reach the lungs, they can thrive in the warm tissue and begin to multiply aggressively.
At this stage, most patients won’t feel anything – but quietly, the disease is already working its way towards becoming extremely dangerous.
It should be noted here that most cases of pneumonic plague do not kill within 24 hours of symptoms beginning. In fact, many take several days.
However, according to the World Health Organisation (WHO), the most severe cases can kill within 24 hours of symptoms starting.
In a severe case, the immune system can become overwhelmed by the sheer volume of bacteria within a matter of hours.
Symptoms take hold
Then, patients begin to experience extreme symptoms.
These include rapidly developing pneumonia – when the infection reaches the lungs – a telltale sign of which is shortness of breath.
Patients will also begin to develop an intense fever and headache, weakness across the body, pain in the chest, and may begin coughing up blood.
With the body’s immune system weakened, the bacteria can begin to destroy lung tissue – rapidly intensifying the pneumonia.
At this point, fluid begins filling the lungs, restricting oxygen from entering and effectively suffocating the patient.
Organ failure and death
Having destroyed lung tissue, the bacteria can then spread to other parts of the body.
This can cause what’s known as secondary septicaemic plague, a severe bloodstream infection.
From here, the bacteria can reach vital organs such as the heart, the liver and kidneys.
And, overwhelmed by the full-body infection, these organs will begin shutting down.
At this point, the body is plunged into a state it cannot recover from – and, ultimately, the patient dies.
The good news
The risk to the wider public remains low, according to the WHO.
A spokesman said: ‘Based on unofficial information available, the public health risk to the general population appears to be low. The risk assessment will be updated once more information is available.’
What’s more, pneumonic plague has not yet been confirmed as the cause of Ms Shipilova’s death.
And so far, everyone who came into contact with her is said to have tested negative for the disease
Crucially, the disease is also treatable with antibiotics, especially if they’re given within 24 hours of symptoms beginning.
