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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

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 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 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

Whether SARS can be contained depends on how well China can control its epidemic. As long as China has cases, or if SARS becomes endemic there, other countries will need to be constantly on guard against imported cases that serve as the seeds for new outbreaks. ''That's the future of the disease,'' Dr. [David L. Heymann] said. ''We don't yet have the data from Hong Kong as to exactly what has happened and what the patients were treated with,'' Dr. Heymann said, though he noted that the SARS virus was identified in some of the relapsed patients. In a few infections, like Ebola, the virus can persist in semen after an individual recovers, but such patients do not suffer relapses. So the W.H.O. is concerned by the reports that a small number of people with SARS might suffer relapses, Dr. Heymann said

In Hong Kong, health officials said it now appeared that many recovered patients may still have some SARS virus in their bodies when they are discharged from a hospital, raising the disturbing possibility that they might still be able to spread the disease. They also confirmed that 12 patients, apparently recovered, had relapsed after their discharge from a hospital. Scientists in Taiwan reported 11 new cases and two additional deaths Thursday. Taiwan's caseload, just 41 as recently as last Friday, has more than doubled, to 89. The scientists said there was new evidence of SARS both within hospitals and in the general community, and added that local authorities were putting large numbers of people into quarantine. Twenty-three people have died of SARS in Toronto, making the city the most seriously infected outside of Asia. There are nearly 150 probable cases in and around the city, and four more in British Columbia. Most of the infected Canadians have recovered. Nearly 40 cases remain active

Falling ill again does not necessarily mean that a patient becomes infectious again. Dr. Brian Tomlinson, a clinical pharmacologist at Prince of Wales Hospital, who has just written a paper about SARS for the British medical journal Lancet, said that relapses, including those that had happened before a patient was discharged, typically represented a late and excessive immune- system response to the virus. Public hospitals in Hong Kong are run as practically independent fiefs, and SARS specialists at Prince of Wales like Tomlinson and Dr. David Hui, the chief of respiratory medicine, said they had been given fairly little information about the relapses. In three-quarters of the more than 300 SARS cases treated at Prince of Wales, the patients' pneumonia has worsened significantly and has spread to both lungs 7 to 10 days after the onset of fever, Hui said. In most of the remaining cases, the pneumonia and fever gradually disappear without the disease ever taking a sudden turn for the worse

Dr. Brian Tomlinson, a clinical pharmacologist at Prince of Wales Hospital, who has just written a paper about SARS for the British medical journal Lancet, said that relapses, including those that had happened before a patient was discharged, typically represented a late and excessive immune-system response to the virus. They do not necessarily include an increase in the actual quantity of the virus present, he said. One source of confusion about the relapses appears to be that they have only occurred among patients of less well- known hospitals. Almost all the initial SARS cases in Hong Kong were treated at Prince of Wales Hospital, the teaching hospital for the Chinese University of Hong Kong. Public hospitals in Hong Kong are run as practically independent fiefs, and SARS specialists at Prince of Wales like Tomlinson and Dr. David Hui, the chief of respiratory medicine, said they had been given fairly little information about the relapses. Hui said a few patients had appeared to recover early, only to fall very ill later. That pattern raises the possibility that a small number of patients may have been discharged from hospitals only to become sick later, Hui said. The relapses have occurred at hospitals that may not be holding patients as long as Prince of Wales, which usually keeps patients for at least three weeks from the onset of fever. In three- quarters of the more than 300 SARS cases treated at Prince of Wales, the patients' pneumonia has worsened significantly and has spread to both lungs 7 to 10 days after the onset of fever, Hui said

The new development is another sign of the critical need for doctors to develop a diagnostic test for SARS, as Hong Kong hospitals have been releasing patients who no longer show symptoms even though the hospitals have no way to determine if the patients still have the virus. Such a test would allow epidemiologists to conduct studies to determine, among other things, how long someone infected with SARS could shed the virus. It might also help determine whether infection with the virus confers long-term immunity or whether individuals are vulnerable to a second attack. The medical expert in Hong Kong said the 12 patients were of a range of ages, and were not only the elderly, who have had many of the most serious cases and have had the highest mortality rates from the disease. Still, 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.)

There were two unlucky misses very early in the epidemic. On Nov. 20, at what is thought to be the very beginning of the outbreak, a W.H.O. flu specialist who was in Beijing for a vaccine conference heard of a spate of unusual deaths among health care workers in Guangdong, which he now suspects included SARS cases. He requested tissue samples, but the ones he received contained only common flu strains. On Nov. 20, Dr. Klaus Stohr was in Beijing to discuss China's flu vaccination policy. Dr. Stohr heads the W.H.O.'s flu program, for which 111 laboratories around the world annually scan 200,000 samples from flu victims, looking for candidates for the next year's vaccine. As it happened, the first case of SARS had emerged that month in Guangdong, perhaps in the markets where all sorts of live animals, including chickens, cats, turtles and badgers, are sold for food. At the meeting in Beijing, Guangdong's representative described a small flu outbreak that had killed several people in one hospital. ''I think that was SARS,'' Dr. Stohr said in hindsight