Two people from China have been struck down with the pneumonic plague, a rare lung infection which is present in rodents.

Key points: The two patients are from north-western Inner Mongolia

The two patients are from north-western Inner Mongolia Authorities said they were working to prevent the spread of the contagious disease

Authorities said they were working to prevent the spread of the contagious disease The plague can be transmitted to humans from infected rats via fleas

The two individuals were being treated at a central Beijing hospital, local authorities said. It is not yet known how they came into contact with the disease.

If left untreated, pneumonic plague can prove fatal within 72 hours and is the "most virulent form of plague," according to the World Health Organisation (WHO).

There are two main types of plague — pneumonic and bubonic.

The Black Death — a disease that wiped out almost a third of Europe's population more than 600 years ago — is thought by some experts to have been pneumonic, not bubonic, plague.

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

The Chinese Government did not respond to requests 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," Fabio Scano, coordinator at the WHO China, said.

Mr Scano said 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's 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.

The WHO states that bubonic is the more common, and less deadly, form of the disease, with sufferers also experiencing swollen lymphs.

'Treatable with antibiotics'

When the news first broke, it was reported that Chinese authorities had sought to "block and control" online discussions about the spread of the infection — however, in the days since, the ABC has seen various discussions taking place on public Chinese social media platforms, like Weibo and WeChat.

Many posts expressed concern about a possible spread, while Chinese state media posted information about the pneumonic plague and advice on protective measures to avoid contracting it.

"I just want to know how these two came to Beijing?" one user posted on Weibo.

"By train, airplane, or did they drive themselves?"

"Bird flu in the year of the rooster … swine fever in the year of the pig," another posted. "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.

Alexandra Phelan, who is an adjunct professor in global health law at Georgetown University in Washington, DC, told the ABC it was not unheard of for cases of plague to occur in China.

"Pneumonic plague can spread to other people and so China will need to be conducting surveillance and tracing possible contacts. It is quite treatable with antibiotics, if treatment is given quickly," she said.

Dr Phelan added that it was a concern only "limited information" was coming out of China about the two cases.

"Under international law, in certain circumstances China should be sharing this information."

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

The marmot is a large type of ground squirrel found across Mongolia and Russia. ( Wikimedia Commons )

In May, a Mongolian couple died from the bubonic plague after eating raw marmot kidney. Marmots, which are a large ground squirrels, are known to be carriers of the plague germ. And in 2014, a man died of the plague in north-western Gansu province in China, sparking the quarantine of 151 people.

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

The Black Death is considered one of the biggest and most devastating diseases in human history.

In Europe in the Middle Ages, an estimated 30 million people — about one in three — were killed by the disease.

Editor's Note — November 15, 2019: An earlier version of this story referenced unattributed reports that Chinese authorities had attempted to control and censor discussions around the infection's spread online. The story has been amended to attribute the reports to the New York Times and highlight that various online discussions have been taking place on China's biggest social media platforms Weibo and WeChat.

AFP/ABC