Darya Shipilova, who worked at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East, died in the intensive care unit of the district hospital in Shelekhov, a town near the regional capital Irkutsk, Euronews reported.

She reportedly told medical staff that she had accidentally broken a test tube containing live Yersinia pestis, the bacterium that causes plague, while collecting samples on Sept. 25, according to The Moscow Times.

Her condition suddenly worsened two days later, and she was admitted on Sept. 29 with symptoms of severe pneumonia and placed on a ventilator.

Around 200 people have since been placed under strict medical observation. Several Irkutsk hospitals have closed for a three-week quarantine, and at least one maternity hospital is also under quarantine.

Pneumonia of unknown cause

Russian authorities have not confirmed plague as the cause of death.

Rospotrebnadzor, the country's public health watchdog, said on Oct. 4 that she had been diagnosed with pneumonia of unknown cause and that the epidemiological situation in Irkutsk and Shelekhov was stable, according to CNN.

The agency said samples taken from the patient showed no microorganisms related to her work and that no incidents involving pathogens had been recorded at the institute.

Irkutsk Governor Igor Kobzev urged calm on Oct. 2 after meeting Rospotrebnadzor chief Anna Popova, saying contacts under observation showed no signs of illness and had tested negative, The Moscow Times reported. He referred only to "a particularly dangerous infection."

Authorities have called off planned public events in Irkutsk, and law enforcement agencies have reportedly opened a criminal case over possible health safety violations that led to a death.

A U.S. State Department spokesperson told CNN on Oct. 4 that Washington was aware of the possible death from pneumonic plague and was monitoring the situation with the Centers for Disease Control and Prevention. "We encourage Russian authorities to share accurate information quickly and openly," the spokesperson said.

Russian media have described the suspected infection as pneumonic plague, The Moscow Times reported.

Pneumonic plague is the most virulent of the three main forms of the disease and can pass between people through respiratory particles, according to the World Health Organization.

Left untreated it is always fatal, but recovery rates are high when the infection is caught and treated within 24 hours of the first symptoms, the WHO says.

Plague, known as the Black Death when it killed more than 50 million people in Europe in the 14th century, is now readily treatable with antibiotics, the agency says.