Beijing — Two people in Beijing have been diagnosed with the pneumonic plague, a rare instance of the highly-contagious disease that is fatal if left untreated. The two individuals were being treated Tuesday at a hospital in China's capital city, which is home to more than 21 million people, local authorities said.

Pneumonic plague can prove fatal within 24 to 72 hours and is the "most virulent form of plague," according to the World Health Organisation (WHO), while the bubonic form is less dangerous.

The patients are from the northwestern Inner Mongolia province, district officials said in an online statement, adding that the "relevant prevention and control measures have been implemented."

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The Beijing government did not respond to AFP's calls for comment, but the WHO confirmed that Chinese authorities had notified them about the plague cases.

"The (Chinese) National Health Commission are implementing efforts to contain and treat the identified cases, and increasing surveillance," said Fabio Scano, coordinator at WHO China.

Scano told AFP that "the risk of transmission of the pulmonary plague is for close contacts and we understand that these are being screened and managed."

According to the WHO website, the lung-based pneumonic plague is very contagious and "can trigger severe epidemics through person-to-person contact via droplets in the air."

Symptoms include fever, chills, vomiting and nausea.

On Weibo, a Twitter-like social media platform, Chinese censors scrubbed the hashtag "Beijing confirms it is treating plague cases" as they tried to control discussions - and panic - around the disease.

"I just want to know how these two came to Beijing??" posted one user. "By train, airplane, or did they drive themselves?"

"Bird flu in the year of the rooster...swine fever in the year of the pig," wrote another. "Next year is the year of the rat...the plague is coming."

The plague germ Yersinia pestis can be transmitted to humans from infected rats via fleas.

A file microscopic image shows Yersinia pestis, the germ that causes the plague. Getty/iStockphoto

In 2014, a man died of the plague in northwestern Gansu province in China and sparked the quarantine of 151 people.

The 30,000 people living in Yumen, the town where the man died, were also prevented from leaving, with police at roadblocks placed on the town perimeter.

According to China's National Health Commission, a total of five people have died from the plague between 2014 and September of this year.

In May, a couple in neighboring Mongolia died of the bubonic plague after eating raw marmot kidney, triggering a quarantine that left tourists stranded in a remote region for days. The ethnic Kazakh couple died in Nogoonnuur soum, which borders Russia and China.

Plague, sometimes known as the Black Death, wiped out millions of people in the Middle Ages during widespread outbreaks the mid-1300s. It is estimated to have claimed the lives of 60 percent of the European population, with some estimates placing the human death toll that century at 200 million.

Though cases are rare today, it is a common misconception that the plague has been wiped out. Cases have even been known to occur in the U.S., transmitted primarily through contact with wild rodents in western states.