Two cases of plague, the illness known for causing the Black Death, have been reported in China, CNN reports.

What we know: The two patients are from the Chinese province of inner Mongolia and were diagnosed by doctors in Beijing. Both individuals are receiving treatment in Beijing's Chaoyang District, where authorities say they've implemented control measures to prevent the infection's spread.

The plague had been detected in the region earlier this year as well when a Mongolian couple died from the disease after consuming raw marmot kidney.

Background: European outbreaks of the plague during the Middle Ages killed roughly 50 million people. Advancements in antibiotics have made the disease fairly treatable, but have not eradicated it entirely. The World Health Organization reported over 3,248 cases of the plague worldwide from 2010 to 2015.

The plague is caused by bacteria and transmitted through infected animals and flea bites. It comes in three forms, including:

Bubonic plague, the disease's most common form, which often causes flu-like symptoms and swollen lymph nodes.

the disease's most common form, which often causes flu-like symptoms and swollen lymph nodes. Septicemic plague , which involves the bacteria entering the bloodstream and multiplying.

, which involves the bacteria entering the bloodstream and multiplying. Pneumonic plague, which infects the lungs and causes serious coughing.

Of note: Pneumonic plague, which the Chinese patients have, is the only form of the plague that can be spread from person to person.

Go deeper: