Around 200 people have been placed under medical observation in Irkutsk in eastern Siberia, Russia, after an employee of an anti-plague institute there died of suspected plague on October 2.According to local reports, Darya Shipilova had broken a test tube containing the plague pathogen. She fell ill with suspected pneumonic plague after returning from a work trip to Buryatia, where an outbreak had been reported near the border with Mongolia.“A special set of measures was organized and carried out following a case involving an employee of the Irkutsk Anti-Plague Research Institute of Siberia and the Far East, who was diagnosed with pneumonia of unknown etiology,” Russia’s consumer watchdog Rospotrebnadzor told the state agency Tass.The watchdog said expanded testing of samples from the patient found no microorganisms linked to her work at the lab.According to its website, the Anti-Plague Institute is a major research and public health facility responsible for monitoring and preventing outbreaks of high-risk bacterial and viral infections across Siberia and Russia’s Far East.First, what is the plague?Plague is an infectious disease caused by Yersinia pestis, a zoonotic bacteria that can spread between animals and people. Y. pestis is usually found in small animals and their fleas.According to the World Health Organisation (WHO), humans can be infected in one of three ways — “the bite of infected vector fleas”, “unprotected contact with infectious bodily fluids or contaminated materials” (like being bitten by an infected rat), and “the inhalation of respiratory droplets/small particles from a patient with pneumonic plague”.Types of plagueStory continues below this adBUBONIC PLAGUE: Bubonic plague is caused by the bite of a flea infected with Y. pestis. The disease travels through the lymphatic system to the nearest lymph node where it replicates itself.Of the three variants, bubonic plague has proven to be especially deadly in the past: The Black Death, the 14th-century plague pandemic, killed more than 50 million people in Europe, according to the WHO — up to one-third of the continent’s population.Also in Explained | Lung cancer is India’s deadliest cancer. So why don’t we screen for it?Symptoms typically include sudden onset fever, chills, head and body aches and weakness, vomiting and nausea. The lymph node becomes inflamed and painful as a result, and is called a ‘bubo’.SEPTICAEMIC PLAGUE: In this variant, the Y. pestis bacteria multiply rapidly in the bloodstream and can develop from any other form of plague. Primary septicaemic plague can also occur.Story continues below this adSymptoms can range from bleeding under the skin to severe complications, including organ failure, tissue damage, respiratory distress and bleeding under the skin. If left untreated, it could turn fatal.PNEUMONIC PLAGUE: Pneumonic plague, which Shipilova was suspected to have, is the most virulent form of plague with an incubation period as brief as 24 hours.Pneumonic plague is the most dangerous, and according to the WHO, “invariably fatal” if untreated. As the name suggests, it happens when the bacteria enter the lungs and causes rapidly developing pneumonia and other symptoms. According to the CDC, it is the only form of plague that can be spread from person to person by inhaling infectious droplets — also making it the most contagious.However, this disease also enjoys high recovery rates if detected and treated within 24 hours of onset of symptoms.What you should knowStory continues below this adWith the advent of modern antibiotics, which can treat Y. pestis, and improved hygiene and understanding of the disease, plague is now relatively uncommon. However, it remains endemic in several countries.Also Read | Heart attacks in fit women in their 30s and 40s: The hidden risk doctors want you to knowToday, plague is concentrated largely in the Democratic Republic of the Congo and Madagascar. Between 2019 and 2025, WHO recorded 3,860 suspected cases worldwide, including 2,646 confirmed cases and 423 deaths among the countries reporting confirmed cases.Plague is treatable with antibiotics, and early treatment greatly improves the chances of survival. Untreated pneumonic and septicaemic plague are invariably fatal.