Local authorities in the city of Bayan Nur in the Chinese region of Inner Mongolia issued a warning on Sunday, one day after a hospital reported a case of suspected bubonic plague.

It followed four reported cases of plague in people there last November, including two of pneumonic plague, a deadlier variant.

“We are monitoring the outbreaks in China, we are watching that closely and in partnership with the Chinese authorities and Mongolian authorities,” WHO spokeswoman Margaret Harris told a UN press briefing in Geneva.

“At the moment we are not … considering it high risk but we are watching it, monitoring it carefully,” she added.

The bubonic plague, known as the “Black Death” in the Middle Ages, is a highly infectious and often fatal disease that is spread mostly by rodents.

Cases are not uncommon in China although they are becoming increasingly rare.