A worker at a Russian plague research institute has died after developing a severe illness, prompting authorities in Siberia to place nearly 200 people under medical observation.

The woman, identified by local media as 28-year-old Darya Shipilova, worked at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. Reports have raised concerns that she may have contracted pneumonic plague, but Russian authorities have not confirmed this.

Cause of death remains unclear

Russia's public health watchdog, Rospotrebnadzor, said Shipilova was diagnosed with "pneumonia of unknown aetiology".

According to Reuters, authorities said testing had not found evidence that her illness was caused by pathogens linked to her work at the institute. Officials have maintained that the situation is under control.

Reports that Shipilova may have been exposed during a laboratory accident have not been officially confirmed.

Al Jazeera reported on Tuesday that Shipilova became ill while at work on September 30 and died two days later at a hospital in Shelekhov.

Authorities have identified and examined people who had contact with her and placed them under medical supervision.

What is pneumonic plague?

Plague is caused by the bacterium Yersinia pestis, which is commonly carried by small mammals and their fleas.

Pneumonic plague affects the lungs and, unlike some other forms of the disease, can spread between people through infectious respiratory droplets.

Microbiologist Thomas Jeffries, writing for The Conversation, explained that the conflicting information surrounding Shipilova's death has fuelled concern, while the exact circumstances remain unknown.

The infection can be treated with antibiotics when detected early but can become fatal quickly without treatment.

WHO says public risk appears low

The World Health Organization said that based on information currently available, the risk to the general population appears to be low. It said further assessments would be made as more information becomes available.

The United States is also monitoring developments, while Russian authorities continue investigating the cause of Shipilova's death.