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Russian authorities are investigating the death of a 28-year-old laboratory worker at the Anti-Plague Research Institute in Shelekhov.
The illness has been described as pneumonia of unknown origin or a dangerous infection of unknown aetiology.
Regional officials have acknowledged concerns about a possible case of pneumonic plague.
Nearly 200 people are under medical observation, and isolation measures have been introduced at a maternity hospital.
Why it mattersA suspected plague death and mass quarantines signal a potential containment failure that could disrupt health services and widen public health risk.
More context
Precautionary steps also include mask requirements at a nearby industrial plant.
Experts say the exact pathogen remains unclear, and official confirmation is pending.