Doctors in Manchester say a British man suspected of contracting the severe flu-like illness which has claimed at least nine lives in Canada and Asia has improved slightly overnight.
Doctors in China have been treating hundreds of cases
However, it could be several days before the exact nature of his illness is confirmed.
The patient, a businessman, who is 64 and lives in Manchester, became seriously ill after returning to the UK from Hong Kong.
Tests are being carried out to find out whether he is suffering from the pneumonia-like condition known as Severe Acute Respiratory Syndrome.
The World Health Organisation (WHO) issued a rare emergency warning at the weekend, declaring the sickness "a worldwide health threat", and saying that cases had been reported on three continents.
Experts believe the failure to isolate the cause of the illness increasingly suggests that a new microbe not previously known to medical science is responsible.
Hong Kong flight
The British patient is being treated in an isolation unit in specialist Infectious Diseases Unit at North Manchester General Hospital.
He has undergone a series of tests.
We have to be concerned about an illness that has caused such serious pneumonia
Professor Sir Liam Donaldson
And while doctors await the results, he is being treated with intra-venous antibiotics and oxygen therapy.
Hospital staff who are in direct contact with the patient are wearing protective clothing including masks, aprons and visors. The patient's immediate family are taking similar precautions.
The man, who has not been named, was seriously ill when he appeared at accident and emergency after getting off a flight from south east Asia.
He was flying from Hong Kong to Manchester via Amsterdam.
Dr Roger Glew, medical director for Pennine Acute NHS Trust, which covers North Manchester General Hospital, said: "He was having severe difficulty breathing yesterday but that has eased.
"He has been able to talk to his wife who is visiting."
Search for treatment
The deaths of a mother and child returning from Hong Kong have been reported in Canada.
One victim died in Hong Kong last week and 83 people have already been taken ill.
Other cases have been reported in Vietnam, where a nurse died on Sunday.
Media interest in the UK case is intense
Other passengers on the British man's KLM flight are being told to go straight to a doctor if they develop the severe flu-like symptoms.
The priority now for scientists is to identify the bacteria or virus which is causing the infection.
Then they can work out how it is transmitted and how to treat it.
England's Chief Medical Officer Professor Sir Liam Donaldson told the BBC: "One of the problems with this illness is that so far all the experts around the world haven't been able to pin down exactly which bacterium or virus causes it.
"The first thought was was this the strain of influenza that everybody has been fearing might come out of the far East and cause another world-wide pandemic.
"So far there has been no matching of the bug that causes this disease to any particular strain of influenza. At the moment it remains a mystery."
Sir Liam said all doctors had been warned to be on the look out for patients with suspicious symptoms.
This was particularly important because the bug appears to be spread by close contact, and a large number of cases have been among health workers who have treated infected patients.
Sir Liam said: "It is possible that it will just burn out and be something for the history books.
"But at this stage we have to be concerned about an illness that has caused such serious pneumonia, and where we have not been able to detect the organism that is causing it."