SHANGHAI (Reuters) – Chinese health officials say the risk of an outbreak of pneumonic plague is minimal after two new cases were confirmed this week in Beijing, the official China Daily said on Thursday.

Two patients from Inner Mongolia were quarantined after being diagnosed with the highly infectious and often fatal disease, health authorities in the Chaoyang district of the capital said on Tuesday.

State news agency Xinhua said one of the two was in critical condition while the other was stable. Infected in Inner Mongolia, they were sent by ambulance to a medical institution in Beijing.

The Chinese Center for Disease Control and Prevention said all those at risk of exposure to plague had been traced and treated, and hospitals had stepped up monitoring of those with similar symptoms.

Pneumonic plague is a lung disease spread mostly by rats, with death rates of close to 100% in humans if left untreated, Beijing health authorities said in a notice.

Outbreaks in China have been rare, but large parts of the northwestern city of Yumen were sealed off in 2014 after a 38-year-old resident died of bubonic plague, known as “Black Death” in the Middle Ages and caused by the same bacterium as the pneumonic variant.

Rodent populations have risen in Inner Mongolia after persistent droughts, worsened by climate change. An area the size of the Netherlands was hit by a “rat plague” last summer, causing damages of 600 million yuan ($86 million), Xinhua said.

(Reporting by David Stanway; Editing by Sam Holmes and Clarence Fernandez)