Image copyright Science Photo Library Image caption Plague affects wild rodents and is then spread by fleas

Part of a city in north-west China has been sealed off and dozens of people placed in quarantine after a man died of bubonic plague, state media say.

The man died in Yumen city, Gansu province, on 16 July.

A total of 151 people have been placed under observation, Xinhua news agency says. Authorities have isolated a part of the city centre and three sections of Chijin town which is an hour away.

The man was believed to have caught the infection after contact with a marmot.

Marmots are large, squirrel-type rodents that live in mountainous areas.

The victim is reported to be a 38-year-old man who had fed a dead marmot to his dog.

The deputy head of the hospital where the man died told reporters that the victim had arrived with an increased heart-rate and seemed to be slipping into shock. The hospital has since been quarantined.

The plague

•The plague is one of the oldest identifiable diseases known to man

•Plague is spread from one rodent to another by fleas, and to humans either by the bite of infected fleas or when handling infected hosts

•Recent outbreaks have shown that plague may reappear in areas that have long been free of the disease

•Plague can be treated with antibiotics such as streptomycin and tetracycline

•Madagascar recently recorded 60 deaths from plague

Source: World Health Organization

It is not clear from reports how big the four quarantine zones are. Ten checkpoints have been set up around Yumen and Chijin.

Those in quarantine all had contact with the man, Xinhua said. None was showing signs of infection, it said.

Officials have told reporters that the group could be released after nine days of quarantine if no further cases of plague appeared among them.

Yumen is a small city in western Gansu province, which borders Xinjiang. The last reported case of bubonic plague in the city was in 1977, Xinhua said.

Gansu has seen at least five cases of the plague in the last 10 years, according to the agency.

Image caption Marmots are large, squirrel-type rodents that live in mountainous areas

Bubonic plague, known as the Black Death when it killed an estimated 25 million people in Europe during the Middle Ages, is now rare.

It is a bacterial disease mainly affecting wild rodents that is spread by fleas. Humans bitten by infected fleas can then develop bubonic plague.

Once bacteria infects the lungs, human-to-human transmission of pneumonic plague can occur through coughing.

If diagnosed early, bubonic plague can be successfully treated with antibiotics, while pneumonic plague has a high mortality rate, the World Health Organization says.