A worker at a Russian anti-plague lab is dead after coming down with a mystery case of pneumonia ... and the fallout has nearly 200 people under medical watch.

The 28-year-old woman worked at the Irkutsk Anti-Plague Institute of Siberia and the Far East, and Russian officials say she died last week from pneumonia of an undetermined origin.

The situation got serious enough that the hospital facility where she was treated was quarantined ... while Russian media reported nearly 200 people who had contact with her were placed under precautionary observation.

Russian officials are trying to tamp down fears, saying no microorganisms tied to the woman’s work were found in her body and no other lab employees have gotten sick.

Still, the U.S. is watching closely ... a State Department spokesperson says there are reports of a fatal suspected case of pneumonic plague and is urging Russia to share information quickly and openly.

Secretary of State Marco Rubio says he doesn't think there's reason to panic ... but warned if something like that gets out, it can spread fast.

The World Health Organization says it's aware of the reports, but no official cause of death has been confirmed.

Pneumonic plague is caused by the same bacteria behind bubonic plague and can spread through respiratory droplets ... though experts say the odds of this turning into a major outbreak are extremely low.

For now, one worker is dead, a hospital is under quarantine and nearly 200 people are being watched ... while the exact cause remains a mystery.