THE plague ravaged across the world hundreds of years ago, claiming the lives of hundreds of millions of people between the 14th and 17th centuries.

Now, health officials in China are treating three people with the disease and warning citizens to protect themselves.

The plague?

It’s an infectious disease caused by a bacteria, Yersinia pestis, usually found in small mammals and their fleas. Humans can be contaminated by either the bite of an infected flea or direct contact with infected materials, or by inhalation.

It’s back?

It has never been fully eradicated. Between 2010 and 2015 there were 3,248 cases of plague worldwide and the disease caused 584 deaths, according to the World Health Organization (WHO).

There’s an outbreak in China?

Three cases have been reported. A middle-aged couple from a rural village, Xilingol League, became infected with pneumonic plague on November 12. A 55-year-old man, from the same village, has become the third person to be diagnosed. He has bubonic plague, after killing and eating a wild rabbit on November 5.

Where are they?

The man is being treated in hospital 200 miles from Beijing, while 28 people who had close contact with him are in quarantine. None have a fever or are displaying any other symptoms.

The couple are quarantined in Beijing. Areas connected to the victims have been disinfected.

Bubonic and pneumonic?

They are the two most common forms. Bubonic - the most common - travels to a lymph node, where it becomes inflamed and causes swollen lymph nodes, “buboes”. Pneumonic is the most severe form and can develop from bubonic plague, resulting in a lung infection, causing shortness of breath, headaches and coughing.

It’s still a killer?

If left untreated, the plague is always fatal.

In May, a Mongolian couple died from bubonic plague after eating the raw kidney of a marmot, a local folk health remedy.

But it’s not the killer it once was?

Historically, it was responsible for widespread pandemics, killing so many millions it became known as the "Black Death”.

Nowadays, it is easily treated with antibiotics and taking precautions to prevent its spread - not travelling to infected areas and not having contact with rodents.

There’s an online blackout?

On Tuesday, the New York Times reported that censors in China instructed “block and control” of online discussions.

Citizens are concerned?

Before the blackout, social media users expressed concern about how the couple arrived in Beijing, noting that if they travelled by public transport, this could have involved contact with many more people.

China blame climate change?

Officials have pointed to climate change as a cause of the rise in rodent populations, with heavier rain and longer summer droughts allowing rats to thrive.

When was the last UK outbreak?

The last major outbreak of bubonic plague was in 1665 to 1666, killing nearly a quarter of London's population. While 68,596 deaths were recorded in the city, the true number was probably over 100,000. Other parts of the country also suffered.

MAUREEN SUGDEN