The death of a lab worker in Siberia has put the spotlight on pneumonic plague, a rare but dangerous form of the disease. Here is what it is.

Plague is caused by a bacterium called Yersinia pestis. It is the same germ behind the medieval Black Death. The pneumonic form attacks the lungs and can spread from person to person through respiratory droplets, like the flu.

Pneumonic plague is highly contagious. One analysis puts its reproduction number between 2.8 and 3.5, similar to early COVID-19 variants. The World Health Organization warns it can be fatal without rapid treatment.

Timing matters. If treated with antibiotics within the first 24 hours of symptoms, the chances of recovery are good. After 24 hours without treatment, the death rate can jump to around 50 percent, according to health experts.

Plague bacteria still exist in nature. They are found in wild marmot populations across Russia, China and Mongolia. Russia’s last registered case was in 2016, when a boy was infected while skinning a marmot.

The Irkutsk Anti-Plague Institute, where the worker was employed, is a biosafety level 3 facility that opened in 1934. Authorities have opened an investigation into whether safety protocols were followed.