Roughly 200 people are under medical observation, several hospitals are under quarantine, and dozens are in isolation in Russia following the unexplained death of an infectious disease researcher at the Irkutsk Anti-Plague Research Institute of Siberia, in the town of Shelekhov, according to early reports of the situation on Sunday and Monday.

Researcher Darya Shipilova reportedly died last week from pneumonia of ‘unknown’ cause, according to Rospotrebnadzor, a Russian public health agency. Unconfirmed information regarding her death has swirled. Some reports have claimed she accidentally broke a test tube containing the plague pathogen, Yersinia pestis, on September 25. She subsequently fell ill, was admitted to a hospital September 29 with severe pneumonia and died overnight between October 1 and October 2. The head of the Russian region that neighbors Irkutsk, Alexey Tsydenov, initially reported that she had died of plague but later altered his statement to say she ‘possibly’ died of the disease.

Y. pestis can cause several forms of plague: bubonic, septicemic, and pneumonic—the latter of which can be aggressive and quickly fatal. However, the pathogen is generally easily treated by antibiotics. In the US, the Centers for Disease Control and Prevention recommends that anyone exposed to the pathogen should undergo an immediate seven-day post-exposure antibiotic prophylaxis treatment. Additionally, though pneumonic plague is spread via respiratory particles from coughing, it is not considered highly infectious; it requires direct, close contact for the bacteria to be transmitted.

The world watches with a mixture of dread and curiosity. Is this the modern-day return of the Black Death, or just a tragic accident in a lab? The Russian authorities have their work cut out to reassure the public and solve the mystery. For now, the plague remains a shadow in the corner of our eyes, a reminder of our vulnerability in an ever-connected world.