The health chief leading the fight against a lethal pneumonia bug has told the BBC he believes a reliable test for the condition will be available in weeks.
Hong Kong is not proving an attractive destination for tourists
The test will be key in helping scientists to learn more about what causes Severe Acute Respiratory Syndrome (Sars), how it spreads and how to stop it.
Dr David Heymann of the World Health Organization said he could not set a definite time for the breakthrough but predicted: "It will certainly be weeks rather than months".
"It's a disease which first and foremost affects health workers who are taking care of patients," he said.
"These are usually very healthy workers - young healthy people - and it then spreads to their family members and then out into the general public."
Sars has killed about 80 people across the world, more than half of them in China, and infected more than 2,000 others.
In the United States, President George W Bush issued an executive order allowing the forced quarantine of patients with Sars if deemed necessary by the health secretary.
The order calls for the "apprehension, detention or conditional release of individuals to prevent the introduction, transmission or spread of suspected communicable diseases".
It adds Sars to a list including cholera, plague, smallpox and Ebola.
The US is investigating 100 cases of suspected Sars across the country.
Its navy has cancelled all port calls and other travel to southern China and Hong Kong, a spokesman said on Friday.
Washington has already offered flights to all non-essential diplomatic staff to leave Hong Kong and China.
But WHO now believes the epidemic in the southern Chinese province of Guangdong appears to be under control while an outbreak in Singapore could be almost over.
In Hong Kong, which is next to Guangdong, health officials say the rate of increase in cases has been levelling off, but Russia has reported its first suspected cases in two men who had been working in China.
Experts have also have found more evidence that the bug can be spread easily.
SARS: PROBABLE CASES AND DEATHS
China 1220 cases (49 deaths)
Hong Kong 761 (17)
Singapore 100 (5)
Vietnam 59 (4)
Canada 62 (7)
Thailand 7 (2)
Source: WHO - April 4 1700 GMT
Five of the 24 Sars cases in the southern Chinese city of Foshan examined so far by a WHO team were caught despite the patient having no obvious contact with an infected person.
Scientists now believe that, in some cases at least, the "infectious agent" involved can be spread either through airborne droplets - or perhaps linger on objects such as door handles.
Researchers first believed that only "close contact" - such as being sneezed on by a sufferer - could spread the infection.
If another route of transmission is confirmed, it could yet make it far tougher to halt the outbreaks in China and elsewhere.
The WHO has already urged people not to travel to the southern province of Guangdong, or to Hong Kong, unless it is essential.
HAVE YOUR SAY
I believe that much of the reporting has been alarmist. Nearly 700 people in Hong Kong have been affected, but that is less than 1/100 of 1% of the population.
Hong Kong has reported a sharp fall in air travel to and from the territory.
Grace Ng, a spokeswoman for the Civil Aviation Department, said that about 10% of flights had been cancelled in the last three days.
The mystery virus has travelled the globe in a matter of weeks, aided in large measure by international air travel.
Tour operators and hotels are also reporting falls in business since the WHO advice was issued on Wednesday.
Alarm over the disease comes as Hong Kong prepares for the important Ching Ming festival on Saturday, where thousands of residents traditionally go to southern China to visit ancestors' graves.