(CNN) Two people in China are being treated for plague, authorities said Tuesday. It's the second time the disease, the same one that caused the Black Death, one of the deadliest pandemics in human history, has been detected in the region -- in May, a Mongolian couple died from bubonic plague after eating the raw kidney of a marmot, a local folk health remedy.

The two recent patients, from the Chinese province of Inner Mongolia, were diagnosed with pneumonic plague by doctors in the Chinese capital Beijing, according to state media Xinhua . They are now receiving treatment in Beijing's Chaoyang District, and authorities have implemented preventative control measures.

Plague, caused by bacteria and transmitted through flea bites and infected animals, can develop in three different forms. Bubonic plague causes swollen lymph nodes, while septicemic plague infects the blood and pneumonic plague infects the lungs.

Pneumonic -- the kind the Chinese patients have -- is more virulent and damaging. Left untreated, it is always fatal, according to the World Health Organization (WHO).

During the Middle Ages, plague outbreaks devastated Europe, killing around 50 million people. Since then, we've invented antibiotics, which can treat most infections if they are caught early enough -- but the plague isn't gone. In fact, it's made a recent comeback.

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