A lab accident in the Irkutsk region of Siberia led to the death of a 28-year-old woman from pneumonic plague last week, according to reports from independent news outlets. Nearly 200 people who might have come into contact with her are said to have been put under observation and several hospitals in the area have been quarantined.

There is no reason to think this is the start of a large outbreak. Plague cases are relatively common worldwide but they do not spread widely because the infection is now easily treated and prevented.

“Plague is not a historical disease,” Zania Stamataki at the University of Birmingham in the UK said in a statement. “Human cases occur every year, including outbreaks of the pneumonic form.”

The term “plague” usually refers to ill health caused by a bacterium called Yersinia pestis, although there is no official confirmation the woman’s death was due to this pathogen. Y. pestis circulates in non-human animals in many parts of the world and infects more than 1000 people every year.

The most common form of plague is bubonic plague, which is caused by flea bites and characterised by swollen lymph glands, or buboes. The bacterium can also infect blood, called septicaemic plague, or the lungs, known as pneumonic plague.

The woman in Irkutsk is said to have died of pneumonic plague two days after being hospitalised. This is the deadliest form of the condition and can spread via the air when infected people cough, for instance. However, measures such as wearing masks and avoiding close contact prevent its spread.

“This can be controlled and stopped with appropriate surveillance and control measures,” Simon Clarke at the University of Reading in the UK said in a statement. Cases are also usually treatable with antibiotics.

The circumstances of the lab accident remain unclear. It is being reported that it was caused by a broken test tube, but if proper procedures for handling dangerous microbes were being followed, this should not have led to an infection. Workers should have been wearing protective equipment, for instance.

There has also been speculation that the woman’s death was caused by a modified form of the plague created for biowarfare, but there is no evidence this is the case. Pneumonic plague is nearly always fatal if not treated promptly, so there’s no reason to think this strain is especially dangerous. What’s more, Y. pestis frequently infects animals and people, so there are legitimate reasons for studying it in labs.

The former Soviet Union did have a large bioweapons programme, which included modifying Y. pestis to make it resistant to antibiotics. Antibiotic-resistant strains have also evolved naturally, and were first detected in Madagascar in the 1990s.

There is a long history of lab accidents leading to infections, deaths and even outbreaks. For instance, in 2007, an outbreak of foot and mouth disease in the UK was traced to labs. In 1979, at least 68 people died of anthrax in Sverdlovsk, Russia, after what the US claimed was the accidental release of anthrax spore from a military facility. There is, however, no evidence the covid-19 pandemic was a result of a lab accident.

While lab-acquired infections with Y. pestis are rare, they are not impossible, Stamataki said. “This incident warrants a full account of what happened. That account matters beyond Russia.”

Russia has repeatedly claimed that Ukraine is developing bioweapons with the help of the US, but there is no evidence for this. In 2024, it was reported that new buildings were being built at a former bioweapons site in Russia called Sergiev Posad-6.