MOSCOW/LONDON — A laboratory worker at a plague research institute in Siberia fell ill and died in undisclosed circumstances on October 2, prompting authorities to quarantine about 200 people and causing public health fears on social media. Here's what we know about the case, which officials have not linked to her work or plague, and about the disease she studied. What happened? Russian media named the victim as Darya Shipilova, a 28-year-old laboratory worker at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. The authorities said she died of an unknown type of pneumonia but that as a precaution they quarantined everyone who had come into contact with her. Unconfirmed reports of how she fell ill have circulated on social media. One version said she had dropped a test tube containing live bacteria at work, a theory rejected by Russia's health watchdog. Another said she became sick after travelling to Thailand. A third said she fell ill after going to an area near the border with Mongolia. There is no evidence in the public domain to support any of those theories. Did