Representational image A broken test tube inside a laboratory is at the centre of a disturbing medical mystery in Siberia.A 28-year-old laboratory worker in Russia's Irkutsk region has died after reportedly being exposed to live plague bacteria, with local reports saying the exposure may have happened when a test tube broke while she was handling samples. Nearly 200 people who may have come into contact with her have since been placed under medical observation, DailyMail reported.Russian authorities have confirmed a plague death in the region, while reports identifying the woman, the laboratory accident and the infection as pneumonic plague have come from Russian media outlets. What happened inside the Siberian laboratory? The woman has been identified by Russian media as Daria Shipilova, a laboratory technician at the Irkutsk Anti-Plague Institute.According to reports cited by The Moscow Times, she told medical staff that she had accidentally broken a test tube containing live bacteria while collecting samples. She later developed severe pneumonia-like symptoms and was admitted to a hospital in Shelekhov, a town near Irkutsk.She was placed on a ventilator but died after several days in hospital, according to the reports.The exact route of infection, however, remains uncertain. Another account suggested she became ill after returning from a work trip to Buryatia. Regional officials have said the woman had not visited Buryatia and that there was no plague outbreak there.That uncertainty matters because this is not simply a story about a laboratory accident. It is also about a disease that can behave very differently depending on which form of plague is involved. Why is pneumonic plague causing concern? Plague is caused by the bacterium Yersinia pestis, which is best known for the Black Death but has never completely disappeared.There are several forms of plague. Bubonic plague, the most common form, typically affects the lymph nodes. Septicaemic plague affects the bloodstream. Pneumonic plague infects the lungs.The last one is particularly concerning because it can spread from person to person through respiratory droplets.And unlike the plague that people associate with medieval Europe, modern plague is not automatically a death sentence. Antibiotics can be effective when treatment begins early. The danger comes when the disease is missed or treatment is delayed.That is why the people who came into contact with the laboratory worker are being watched closely. Nearly 200 people are being monitored Reports say at least 197 people who may have had contact with the woman have been placed under medical observation. More than 100 were reportedly being monitored in hospital wards.The Shelekhov district hospital was also placed under quarantine while officials assessed the situation.So far, officials have said the identified contacts have shown no signs of illness and have tested negative, according to Irkutsk Governor Igor Kobzev. Public events in the region were also cancelled as authorities worked on containment measures.