U.S. and European officials are tracking a situation in southeastern Russia after a laboratory worker at an institute that studies the plague suddenly died of an unknown illness.

A spokesperson for the State Department said it was "aware of ​reports of a fatal case of suspected pneumonic plague" in Russia and that it was monitoring developments alongside the the Centers for Disease Control and Prevention and other agency partners.

The worker, a woman in her late 20s, was an employee at the Irkutsk Anti-Plague Institute of Siberia and the Far East, which studies the plague and other infectious diseases, according to Rospotrebnadzor, a health and consumer protection agency in Russia.

Kremlin spokesman Dmitri Peskov offered few details about the potential case of the plague in Irkutsk during a briefing with reporters on Oct. 5., saying that Rospotrebnadzor was monitoring the situation.

Local officials said that authorities have implemented "anti-epidemic measures" in the region to contain the infection.

A spokesperson for the European Commission said on Oct. 5 at its regular midday briefing that European authorities are keeping an eye on the situation in Russia.

"We are aware of the reports concerning the laboratory incident in Russia, seemingly involving a pathogen-causing plague," Commission spokesperson Eva Hrncirova said. "We are in touch with our experts from European Center for Disease Prevention and Control."

What's the situation in Russia?

Russian authorities said on Oct. 4 that the situation in Irkutsk is stable after the death of a laboratory worker prompted local officials to impose a "special set of measures" in an effort to contain a dangerous infection.

Rospotrebnadzor has described the illness that caused the woman’s death as "pneumonia of unknown origin."

The research institute at the center of the incident is located in the Siberian region of Irkutsk, roughly 150 miles from Russia’s border with Mongolia.

Igor Kobzev, the governor of Irkutsk Oblast, said on Oct. 2 that authorities identified dozens of people who came into contact with the laboratory worker. They were subsequently placed under medical observation, according to the Russian state news agency Tass.

People in quarantine have not shown any signs of infection as of Oct. 5, according to Maxim Modin, mayor of the nearby town of Shelekhov. He said that the town’s medical facilities have reopened for normal operations and are accepting patients.

"All individuals who came into contact with the woman have been identified, examined, and are under medical observation with daily laboratory tests," he said in a social media post.

The State Department urged Russian officials to be more transparent about the situation in Irkutsk."Many details have not been confirmed," the State Department spokesperson said. "We encourage Russian authorities to share accurate information quickly and openly."

How dangerous is the plague?
It is dangerous, but the plague can be treated effectively with common antibiotics if it is detected in time.

The plague is an infectious disease caused by the Yersinia pestis bacteria, which can be found in wild rodents, small mammals and their fleas.

If untreated, the plague can have a fatality rate of 30% to 100% if left untreated, according to the World Health Organization. There are three main forms of the plague: bubonic, septicaemic and pneumonic.

The most dangerous form of the illness is the pneumonic plague, which can be transmitted person-to-person and can cause severe epidemics.

The illness is infamous for killing tens of million in Europe and Eurasia in the 14th century during a global pandemic known as the Black Death.

In modern times, the plague is a very rare illness that does occasionally infect people in certain parts of the world, such as the western United States and certain regions in Asia and Africa.