It is one of the deadliest infections known to medicine – capable of destroying the lungs, causing vital organs to shut down and killing even a healthy person within days.
And while most people associate plague with the Black Death hundreds of years ago, the disease still exists today.
The deadliest form of plague, known as pneumonic plague, is back in the spotlight after the death of a Russian anti-plague laboratory worker last week sparked panic over the cause.
Initial reports said that Darya Shipilova, a 28-year-old worker at the Irkutsk Anti-Plague Research Institute in Siberia, became ill after possibly contracting the plague bacteria in a lab accident. Russian authorities said she died from ‘pneumonia of unknown origin,’ and the World Health Organization said it was told that no plague had been detected.
The Centers for Disease Control and Prevention (CDC) wrote on X this week that the agency is ‘closely monitoring reports of a suspected case of pneumonic plague in the Irkutsk region of Russia that resulted in a death.’ It also assured there was ‘no indication of a broader threat to the United States.’
Around 200 people and five hospitals, including a children’s and maternity hospital, have reportedly been quarantined as a precaution, while neighboring Kazakhstan, Kyrgyzstan and Uzbekistan have also introduced precautionary measures, including heightened health checks and tighter border controls.
Unlike the more familiar bubonic plague, which is usually spread by infected fleas, pneumonic plague attacks the lungs and can pass directly between people through droplets released when an infected person coughs. Without prompt treatment with antibiotics, it is almost always fatal – and patients can go from feeling unwell to fighting for their lives with frightening speed.
But what exactly happens inside the body to make the infection so deadly – and how quickly can things go wrong? Infectious disease experts have explained to the Daily Mail how the plague spreads throughout the body, from the first hours of infection to the potentially deadly chain of events that follows.
Anti-plague experts at Irkut Anti-Plague Institute are seen in an undated photo
The death of 28-year-old lab worker Darya Shipilova has sparked panic over what caused it
The first minutes and hours: Bacteria take hold
Infection begins when tiny droplets containing the plague bacteria, known as Yersinia pestis, are breathed into the lungs.
‘When this organism gets into the lungs, it’s very happy to be in a nice, warm, moist environment of your lungs,’ said Dr Nicole Iovine, an infectious diseases expert at the University of Florida.
Normally, the body’s immune cells would spring into action to destroy the invading bacteria. But plague has a clever way of stopping them.
‘This is where the organism gets very sneaky,’ Dr Iovine said. ‘It has ways of preventing our immune cells from killing it.’
The bacteria inject proteins into immune cells that interfere with their ability to fight infection – and prevent them from sending warning signals to the rest of the immune system.
‘It basically turns off the house alarm so that the cell doesn’t tell anybody else that there’s a big problem here,’ Dr Iovine explained.
With the body’s defenses temporarily weakened, the bacteria can begin multiplying inside the lungs. Over the course of a few hours, Iovine continued, the bacteria are ‘replicating’ while ‘still preventing our immune cells from calling in the cavalry.’
At this early stage, the infected person will usually have no idea anything is wrong. They may feel perfectly well, even as the bacteria begin to establish themselves in the lungs.
6 to 12 hours: It spreads deeper into the lungs
As the hours pass, the bacteria continue multiplying and can spread deeper into the lungs, reaching the tiny air sacs that allow oxygen to enter the bloodstream.
‘That bacteria is getting into the lower respiratory tracts, down to the bronchi, to the bronchioles, and down to the alveoli,’ said Dr Tyler Evans, an infectious disease expert and author of Pandemics, Poverty and Politics.
These tiny air sacs, known as alveoli, are where oxygen passes into the blood to be carried around the body, while carbon dioxide moves in the opposite direction to be breathed out.
As the infection develops, the bacteria can begin damaging these delicate structures, making it harder for the lungs to do their job.
But at this early stage, the infected person may still feel perfectly well, unaware of what is happening inside their body.
12 to 24 hours: The body fights back
As the bacteria continue to multiply, the body’s immune system eventually realizes something is seriously wrong.
‘After several hours of hiding and replicating, could be six hours, could be 12 hours, you reach so much bacteria in the lungs that finally our immune systems do detect it,’ Dr Iovine said.
‘When they do, it becomes a three-alarm fire because there’s so much bacteria there.’
The immune system launches a powerful attack, sending large numbers of infection-fighting cells into the lungs.
But this response can also damage healthy tissue, causing inflammation that makes it increasingly difficult for oxygen to pass into the blood.
This is the beginning of the dangerous process that, in severe cases, can leave patients struggling to breathe as their lungs fill with fluid.
As the bacteria continue to multiply, the body’s immune system eventually realizes something is seriously wrong
Around 24 hours: The lungs fill with fluid
As the body fights back against the bacteria, the resulting inflammation can cause serious damage to the lungs.
The tiny air sacs that normally fill with air begin to fill with fluid and immune cells instead, making it increasingly difficult for oxygen to reach the bloodstream.
‘You get tons of fluid start pouring into your lungs,’ Dr Iovine said. ‘The lungs are supposed to help you exchange oxygen and carbon dioxide, but if your lungs are full of fluid, they can’t do that.’
As the infection worsens, patients can develop a high fever, chest pain and severe breathlessness. Some may even begin coughing up blood.
‘Around this 24-hour mark… this is when you’re going to get your fever. You’re going to get a lot of shortness of breath. People will cough. They can even start coughing up blood,’ Dr Iovine said.
‘This is the point where people are getting very sick very, very fast.’
With the lungs struggling to get enough oxygen into the blood, the infection can quickly become life-threatening.
24 to 36 hours: The body begins to falter
As the infection worsens, the disease can take an even more deadly turn – spreading beyond the lungs and threatening the entire body.
In severe cases, plague bacteria can enter the bloodstream, allowing the infection to spread to other organs.
‘Sometimes the bacteria can break through blood vessels and get into the blood,’ Dr Iovine explained.
At this stage, patients are at risk of developing septic shock – a life-threatening reaction to infection that causes blood pressure to fall dangerously low, starving vital organs of the blood and oxygen they need to survive.
The immune system, which has been fighting the bacteria in the lungs, can now launch a much wider response.
‘The immune cells of your whole body now are responding to bacteria that were at first only in your lung but are now spread throughout your body because they get into the bloodstream,’ Dr Iovine said.
As inflammation spreads, blood vessels can become leaky, allowing fluid to escape into surrounding tissues.
‘A lot of fluid starts to leak into your tissues, and that means the fluid’s not in your bloodstream anymore. That means then your blood pressure gets low.’
The body now faces two potentially deadly problems at once.
The damaged lungs are struggling to get enough oxygen into the blood, while falling blood pressure means that even the oxygen that does get through may not reach the organs that desperately need it.
Meanwhile, bacteria that have entered the bloodstream can travel to other parts of the body.
‘Once it’s in the blood, it can go to all of your organs, your heart cells, your kidney, your liver,’ Dr Iovine said.
‘Then the immune cells will go to those organs as well to try to kill the infection, but will cause again, the same dysfunction in the organs.’
As the body’s circulation begins to fail, the heart, kidneys, liver and other vital organs may no longer receive enough oxygen to function.
‘Once your blood pressure starts to get very, very low… you can’t even get enough blood to your organs for them to work,’ Dr Iovine explained.
Eventually, cells begin to die because they are deprived of the oxygen and nutrients they need.
‘They can’t get blood, which means they can’t get the oxygen in the blood and the cells will just die,’ Dr Iovine said. ‘That’s what shock will do to you.’
Without urgent treatment, the patient can develop multiple organ failure – a potentially fatal condition in which several of the body’s vital systems begin shutting down.
Pictured above is Yersinia pestis bacteria, which causes plague
36 hours: The infection becomes fatal
By this stage, a patient with severe, untreated pneumonic plague may be fighting for their life.
With the lungs filling with fluid and blood pressure falling dangerously low, the body can no longer get enough oxygen to keep its vital organs working.
‘Your lungs are so full of fluid, you essentially drown in your own secretions,’ Dr Iovine said. ‘It’s really horrendous.’
Without treatment, pneumonic plague is almost always fatal – and even otherwise healthy people can die from the infection.
‘Because it progresses so quickly, unless the antibiotics take place at an early stage, it’s really just too far gone,’ Dr Evans said.
However, antibiotics such as gentamicin can be highly effective if given promptly, ideally within 24 hours of symptoms appearing.
The difficulty is that the illness can worsen so rapidly that there may be little time between the first warning signs and a patient becoming critically ill.
Fortunately, plague is extremely rare in most parts of the world.
In the US, the CDC records an average of seven human plague cases each year, most commonly in western states including New Mexico and Arizona.
More than 80 percent are bubonic plague, usually caught through the bite of an infected flea, rather than the pneumonic form that can spread between people.
‘We see basically a handful of cases every year in the US, and almost none of it sort of progresses to pneumonic. But again, this is globally, this is still very much a real thing,’ Dr Evans said.
Those most at risk of catching pneumonic plague are people in close contact with someone who is infected, particularly before the disease has been diagnosed.
‘The people who are at risk are those who are taking care of a patient who has pneumonic plague,’ Dr Iovine said. ‘That would be family members and potentially healthcare workers who before they recognize that this person has plague.’
People with weakened immune systems or existing lung conditions may also be more vulnerable to serious illness.
But experts stress that the risk to the wider public remains extremely low.
‘This cannot become a pandemic like COVID did,’ Dr Iovine said. ‘And it is not something that the average person needs to be concerned about.’
