A laboratory worker at a plague research institute in Siberia has fallen ill and died in undisclosed circumstances, prompting authorities to quarantine dozens of people and the head of Russia’s health watchdog to fly to the Irkutsk region.

The death has sparked public health fears on social media, but the authorities said on Monday the situation was under control.

Plague is endemic to Siberia, home to the first known prehistoric human outbreak, because the bacterium that causes it, Yersinia pestis, is present among small wild mammals and their fleas in the region’s vast wilderness.

It is rare for Russians to fall ill with plague, however, though cases in other parts of the world — notably Africa — occur every year.

Local media named the woman as Darya Shipilova, an employee at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. There were conflicting reports about how and where the laboratory worker, whose age was given as 28, had fallen ill, but the authorities said she had died of pneumonia of as-yet unestablished origin.

US Secretary of State Marco Rubio said Washington is closely monitoring the case involving a laboratory worker who died at a plague research institute in Siberia.

“We’re watching and monitoring it closely. I don’t think it’s cause for alarm,” Rubio told reporters before leaving for a trip to Europe.

Authorities say no new cases reported, all contacts traced

Russia’s health watchdog, Rospotrebnadzor, told state media the institute had not found evidence of any accidents involving the handling of pathogens after rumours spread on social media that the woman had dropped a test tube containing live bacteria.

Nevertheless, Anna Popova, the head of Rospotrebnadzor, flew to Siberia for an emergency meeting over the weekend.

An unspecified number of people who had been in contact with the dead woman were quarantined, parts of the hospital in the town of Shelekhov where she died were cordoned off and an aluminium plant told its workers to wear masks.

Authorities urged people to remain calm and said there was no evidence that the woman’s death was linked to her work.

Igor Kobzev, the governor of the Irkutsk region, said in a statement that she had died of “pneumonia of unknown aetiology”, adding: “Importantly, over the past few days, no new cases of staff from the Anti-Plague Institute seeking medical attention have been identified.”

He said the situation was under control and that all known contacts of the dead woman had been identified, examined and placed under medical supervision.

Plague comes in three forms affecting different parts of the body. Pneumonic plague, an infection of the lungs, prompts the patient to cough out infectious airborne droplets.

If identified quickly, it can be treated with antibiotics. But if left untreated, it can be fatal in one to six days.