GCXZ

The disease is spread by people coughing or sneezing and can cause breathing difficulties, lung damage and pneumonia.

"We still don't know exactly what killed him," said Dr Jon Bible, a virus expert at Guy's and St Thomas'.

"In the end his lungs were worn down, so secondary infection is the real problem. The virus effectively turns your lungs to jelly."

Known cases of the illness have quadrupled since April, and it is deadlier than Sars, which killed 774 people in 2003. SARS killed one in ten affected people; Mers has proved fatal in 65 per cent of cases.

The majority of the cases have been in Saudi Arabia, or in patients who have recently travelled to the region.

But with the annual Hajj pilgrimage due in October, and an estimated three million people travelling to Mecca, concerns are mounting that the deadly virus could spread swiftly.

"We need to get the facts clear and get the appropriate advice to all your countries where your pilgrims want to go to Mecca," said Margaret Chan, head of the World Health Organisation, in May. "It is something quite urgent."

Still unknown to scientists is how the Mers virus emerged, how people are being exposed to it and whether the contagion is growing. So far, the leading theory is that Mers likely originated in bats and spread to humans.

Cases have been reported in Jordan, Qatar, United Arab Emirates, Britain, France, Italy, Tunisia and Saudi Arabia – the country with the most confirmed infections.

Belgian medics have warned Muslims not to travel to Mecca, because the Hajj threatens to spread the virus worldwide. Doctors expressed concerns that the mass gatherings are "ideal hotbeds for respiratory diseases" and that numbers of Mers cases are greater than actually reported in Saudi Arabia.

Doctors in Saudi Arabia say they are monitoring the disease closely.

"This is really a new phenomenon that we're dealing with," said Keiji Fukuda, WHO assistant director general for health security, at a Geneva conference on infection last month.

"We don't know what the potential is yet, based on the information we have, for sustained human-to-human transmission. We don't know what the full geographic extent of this virus is right now."

The unnamed man in London had been treated in a "negative pressure room" – an isolation chamber within the hospital, which means that contaminated air is not pumped back out into the hospital. All doctors treating the patient would have been equipped with protective suits, which were incinerated after use.

He had initially shown signs of improvement, but his condition deteriorated and he passed away last week. It is unclear how he contracted the disease, although there are known cases in Qatar.

A conference is due to be held in London on Tuesday to discuss how to handle the spreading disease.

"This case shows that we are dealing with an extremely dangerous virus," said Gregory Hartl, spokesman for WHO.

"This disease is slower-burning than SARS, but we don't yet know what to do about it."