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Health officials had assumed that this must be the case because of the pattern of spread in an apartment complex and hotel in Hong Kong. The new findings strengthen the theory, said Dr. Klaus Stoehr, a German virologist and epidemiologist who is the scientific director of the WHO's SARS investigation. Hong Kong officials have found that 203 people with such contacts who were in quarantine developed SARS, 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 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

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

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

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

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.)

May Day 2003. Visitors walk through an almost deserted Tiananmen Square on Thursday in Beijing. The SARS scare kept people away. EUGENE HOSHIKO/THE ASSOCIATED PRESS May Day 2002. Thousands of Chinese throng in front of a huge portrait of Chinese hero Sun Yat-sen in Beijing's Tiananmen Square. NG HAN GUAN/THE ASSOCIATED PRESS

''We believe that Canada's doing the right things and they should have no further setbacks,'' said Dr. David L. Heymann, the W.H.O.'s executive director for communicable diseases, who was in Toronto today for a two-day conference on SARS. 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, said Dr. David Hui, the chief of respiratory medicine. In most of the remaining cases, the pneumonia and fever gradually disappear without any sudden turn for the worse. In what the W.H.O. said was a major change in its definition of a SARS case, the agency added findings from certain laboratory tests as criteria for diagnosing probable cases. The agency recommended that people who test positive be treated in isolation as a measure to bring greater consistency and precision in controlling the disease

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, with nearly 40 cases that remain active. The sequences -- biological blueprints of the virus -- were published by the journal Science after the work was reviewed and authenticated for accuracy by experts. A team of Canadians first sequenced a strain of the virus using specimens taken from a patient in Toronto. Another form of the virus, called the Urbani strain, was sequenced shortly afterward by a U.S.-led team

In Hong Kong, health officials said it appeared that many recovered patients might 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. In Toronto, officials said the two new possible cases did not mean that SARS had crept back into the general community, where no new cases have been reported since April 9. The officials said there was no indication the infected workers spread the disease before they were isolated. Twenty-three people have died of SARS in Toronto, making the city the most seriously infected outside 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, with nearly 40 cases that remain active