India, Oct. 6 -- A laboratory worker in Siberia has died after developing severe pneumonia symptoms, prompting authorities to place nearly 200 people under medical observation over possible exposure to pneumonic plague.

The 28-year-old worker at the Irkutsk Anti-Plague Research Institute reportedly said she had accidentally broken a test tube containing live bacteria while collecting samples. However, Russian health authorities have not confirmed that she had plague and have described her illness as pneumonia of unknown origin.

So, what is pneumonic plague, how does it spread, and why can even a suspected case trigger an immediate public-health response?

In this video, Down To Earth explains what is known about the case, the risks pose...