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Hong Kong officials have found that 203 people with such contacts who were in quarantine developed SARS, [Klaus Stoehr] said. A large proportion of them had had contact with people who were infected at Amoy Gardens, an apartment complex where a large outbreak occurred. Hong Kong officials have theorized that the outbreak resulted from a sewage leak. The SARS virus is quite sensitive to changes in temperature, according to researchers at the National Institute of Infectious Diseases in Tokyo. At body temperature, the SARS virus survived less than four days, the Japanese researchers found. But it survived at least four days at refrigerator temperatures and seems to be able to survive indefinitely at temperatures in a deep freezer. On plastic surfaces at room temperature, the virus could survive for two days, researchers said. Meanwhile, less than a month after scientists cracked the genetic code of the SARS coronavirus, the virus is mutating, causing new forms of SARS to emerge in Southeast Asia

The SARS virus is quite sensitive to changes in temperature, according to researchers at the National Institute of Infectious Diseases in Tokyo. At body temperature, the SARS virus survived less than four days, the Japanese researchers found. But it survived at least four days at refrigerator temperatures and seems to be able to survive forever at temperatures in a deep freezer. On plastic surfaces at room temperature, the virus could survive for two days, researchers said. The SARS virus is a new member of the coronavirus family, which includes viruses that can cause the common cold in humans and many more serious diseases in animals. SARS researchers have tried to extrapolate findings from known viruses in dealing with SARS. Scientists had known that infected individuals could excrete the SARS virus (at least in the RNA, or an immature form) for up to 30 days after onset of symptoms. But those findings reflected continuous production of the virus in the body. The new findings pertain to a significant difference -- stool outside the body

Among the many unknowns about SARS is the incubation period, how long someone can transmit the SARS virus after becoming infected. W.H.O. uses a 20-day period: twice the 10 days that is the longest known interval between exposure to the virus and onset of symptoms. Yet the incubation period may have been as long as 13 days in one case. If 26 days turns out to be a more accurate time span for SARS, then the W.H.O. will adjust accordingly, said Dr. David L. Heymann, who is in charge of communicable diseases for the agency. ''The question is: Is SARS already endemic in China?'' said Dr. Heymann, who said the W.H.O. had not determined the criteria for declaring SARS endemic in a country

The relapses may mean that patients still can transmit severe acute respiratory syndrome, or SARS, even when they are thought to be no longer infectious. It also could be a complication of treatment, perhaps from the use of steroids that so suppressed the patients' immune system that they had no chance to develop a strong immune defense against SARS. The World Health Organization said on Wednesday that it had not yet received reports of the relapses. Dr. Mark Salter, a medical officer with the agency, said it was monitoring the nearly 2,500 SARS patients worldwide who have been discharged from hospitals and had seen no reports of relapses or recurrences. Many SARS patients in Hong Kong are prescribed steroids after they are discharged from the hospital. Doctors here are increasingly concerned that heavy doses of steroids may be suppressing the symptoms of the disease without getting rid of the virus, the expert said

In an ideal world, she said, scientists would try to find out whether the SARS virus was confirmed in the 12 patients when they were first hospitalized and whether it could still be identified after their relapses, or whether the patients had developed one of the many respiratory ailments other than SARS. The new development is another sign of the critical need for doctors to develop a diagnostic test for SARS. Such a test would allow epidemiologists to conduct studies to determine, among other things, how long it would take someone infected with SARS to shed the virus. The number of relapses so far is small - just 12 of the 791 patients discharged from hospitals in Hong Kong, including 39 on Wednesday. (Forty-five percent of the 1,432 surviving SARS patients, or 641, remain hospitalized.)

The pronouncements the first favorable news about severe acute respiratory syndrome since the United Nations agency issued a global health warning on March 12 raise questions about the degree of confidence anyone can have in making predictions about the course of a new disease, a disease about which, as the agency is the first to acknowledge, so little is known. 'We are sailing a boat while we are building it,' Klaus Stoehr, the scientific director of the agency's SARS investigation, said in an interview. among the many unknowns about SARS is for how long someone can transmit the SARS virus after becoming infected. The UN health agency uses a 20-day period: twice the 10 days that is the longest known interval between exposure to the virus and onset of symptoms. Yet the incubation period may have been as long as 13 days in one case. If 26 days turns out to be a more accurate time span for SARS, then the agency will adjust accordingly, said David Heymann, who is in charge of communicable diseases for the agency

In an ideal world, she said, scientists would try to find out whether the SARS virus was confirmed in the 12 patients when they were first hospitalized and whether it could still be identified after their relapses, or whether the patients had developed one of many respiratory ailments other than SARS. None of the relapses or recurrences in Hong Kong have occurred among SARS patients hospitalized in the first wave of infections, when doctors tended to delay prescribing steroids until a patient's pneumonia worsened. The delay might have allowed enough time for a patient's immune system to begin fighting off the SARS virus before steroids began suppressing the immune defenses. Taiwan banned visitors from Hong Kong and other SARS-affected areas over the weekend, while some Shanghai neighborhoods have reportedly begun enforcing quarantines this week on recent arrivals from Hong Kong, Beijing and other areas with outbreaks. The Shanghai restrictions, which have not been adopted citywide, come even after Southeast Asian nations and China agreed on Tuesday in Bangkok not to restrict the freedom of movement of citizens

In an ideal world, she said, scientists would try to find out whether the SARS virus was confirmed in the 12 patients when they were first hospitalized and whether it could still be identified after their relapses, or whether the patients had developed one of the many respiratory ailments other than SARS. Many SARS patients in Hong Kong are prescribed continuing doses of steroids after they are discharged from the hospital. Doctors in Hong Kong are increasingly concerned that heavy doses of steroids may be suppressing the symptoms of the disease without getting rid of the virus, the expert said. The number of relapses so far is small -- just 12 of the 791 patients discharged from hospitals in Hong Kong, including 39 yesterday. (Forty-five percent of the 1,432 surviving SARS patients, or 641, remain hospitalized.)

Yesterday, two top officials of the World Health Organization refused to concede defeat and expressed hope that SARS could be contained if all countries maintained vigilance for the disease. SARS has to be viewed as a worldwide disease, not as one of any country, because its threat is so great to health care workers and systems in affected countries, the W.H.O. said. Dr. [David L. Heymann] also said the W.H.O. was deeply concerned about the possibility that SARS could become an added serious health threat in areas of Africa or Asia where AIDS is highly prevalent. The concern is that SARS could become another so-called opportunistic infection among the millions of people with weakened immune systems from the AIDS virus. Vietnam, which was one of the first countries affected by SARS, may soon become the first country to contain SARS despite its rudimentary health care system, the W.H.O. said. Vietnam has reported 63 SARS cases, with 5 deaths

Also, for unknown reasons, the proportion of recent cases that are testing positive for the SARS virus is declining and a number of people who are not suspected of having SARS are testing positive, said the director, Dr. Frank Plummer. Plummer described his team's findings as 'weird.' He said that they had the potential to weaken the link between SARS in Canada and a previously unknown member of the coronavirus family that the World Health Organization announced last week was the cause of SARS. Plummer said he was surprised to find the virus in 20 per cent of an additional 250 people who were not suspected of having SARS but who were tested because they had returned to Canada from affected areas in Asia or who had mild symptoms not thought to be SARS