Experts believe it is increasingly likely that the bug responsible for a deadly form of pneumonia which has spread around the world is a new microbe not previously known to medicine.
Officials are urging people not to panic
Scientists from 10 countries are now involved in a global detective hunt to find the cause of the illness, dubbed Severe Acute Respiratory Syndrome (SARS).
But they have not been able to pin down the cause of the disease - which is so far thought to have claimed 10 lives.
Iain Simpson, a spokesman for the World Health Organization (WHO) said on Tuesday that the longer the disease continued to baffle scientists, the more likely that it was caused by a new organism.
"If it is something we already knew about we would almost certainly have identified it," he said.
Airports and airlines around the world have begun screening passengers, and refusing entry to those displaying symptoms of the disease, Reuters news agency has reported.
Early signs of the illness include a high fever, chills, a cough and breathing difficulties. It has an incubation period of two to seven days.
The WHO, which has described the illness as a "worldwide health threat", issued a rare emergency warning over the weekend, saying that cases had been reported on three continents, with more suspected in other parts of the world.
Worst affected countries
Hong Kong - 111 cases
Vietnam - 54 cases
Singapore - 23 cases
Some experts believe the condition originated in China's Guangdong province, where five people died and hundreds were infected.
Chinese authorities say that the outbreak is now under control within its borders. There had been rumours that cases had been found in other parts of the country, including Beijing and Shanghai.
Internationally a further five people have died and around 200 been infected by the illness, which is being passed on by air travellers.
Suspected cases have been seen in Israel, Australia, Germany, the US and the UK.
Authorities in Geneva said that two people were in isolation in a hospital after exhibiting symptoms similar to those of the disease.
A British man returning from Hong Kong has been admitted to hospital with a suspected case - the UK's first.
In Australia two women who had recently travelled to China have been hospitalised with symptoms, French news agency AFP reported, although doctors stressed there was no proof they were suffering from the illness.
A British expert says that it is unlikely that the illness is flu - the most frightening scenario for experts.
Professor John Oxford, a world-leading figure in virology, based at Queen Mary's College in London said that the fact that a culprit had not yet been identified was good news.
He said: "If it was influenza, I expect we would have heard this by now. That's certainly rather reassuring.
"This has been smouldering around the Far East for some time - it's not as if it has just exploded, I wouldn't get too worried."
The WHO has warned travellers and airport staff to be on the look-out for signs of the condition.
The majority of cases so far have been in China, Hong Kong and Vietnam.
The illness is thought to be highly contagious
On Sunday, the WHO said two deaths had been reported in Canada. The two are said to be a mother and child returning from Hong Kong, where 123 people have already been taken ill with 111 of those confirmed as having severe pneumonia.
In Germany a doctor suspected of having caught the virus whilst in Singapore is currently in quarantine.
There are also reports of suspected cases of the disease in Switzerland, France, Slovenia, Taiwan, Thailand, China and the Philippines.
However the East Asian region is the worst affected by the disease.
In addition to cases reported in Hong Kong and Vietnam, authorities in Taiwan and Singapore have reported cases of the disease.
No figures are available yet from China, but the WHO said the flu-like symptoms of the disease were similar to those of a sickness in the southern province of Guangdong last month which infected 305 people.
"All we can tell you right now is that the disease situation here has been placed well under control," a Chinese government spokeswoman said on Monday.