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If you have to get sick, you might as well do it in Singapore. The Lion City state's public health-care system is one of the best in Asia, and its government-mandated obsession with hygiene borders on the compulsive. When the SARS epidemic first struck a month ago, Singapore earned praise for its decisive response of quarantining up to 1,500 close contacts of SARS victims, even installing video cameras on their doorsteps to discourage excursions. Singapore's ring-fence approach seemed to work, as the number of new cases dropped to a daily handful—supporting early World Health Organization (WHO) statements that the spread of SARS, as dangerous as it was, could be stemmed with vigilant infection controls.

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But SARS is a slippery bug, and even the tightest ring fence has its holes. By the end of last week Singapore was fighting off a new hospital outbreak, while in Hong Kong new cases continued to mount at an alarming rate, prompting the government there to belatedly begin quarantining household contacts of SARS patients. Though the WHO announced Friday that the epidemic was mostly under control outside Asia, within the region many officials and scientists fear that SARS may be here to stay. "We believe that every citizen could become a carrier of the virus," said Dr. Leung Pak-yin, Hong Kong's Deputy Director of Health.

As with so much of this new disease, scientists still don't know why SARS seems to spread at varying rates in different areas, but this might be the result of the influence of so-called superspreaders, infected individuals who appear to be much more contagious than the average SARS patient. Almost 100 SARS cases in Singapore can be traced to a 26-year-old flight attendant who imported the disease from Hong Kong. In that city, the two major outbreaks—in the Prince of Wales Hospital and the Amoy Gardens housing estate—were almost certainly triggered by a pair of superspreaders. "We don't know how SARS spreads," says Dr. Balaji Sadasivan, Minister of State at Singapore's Ministry of Health, "but we have drawn the conclusion that some people don't seem to spread it to many people while others spread it to a lot." Worried health officials know that each of Asia's supercities is a single superspreader away from another outbreak.

That means identifying potential superspreaders is vital to halting SARS—except that scientists don't know what makes a superspreader super. "That's the $64,000 question," says Dr. Osman David Mansoor, a WHO scientist. "It's probably a spectrum of biological variation." That means genetic aberrations within patients—which could help explain why some SARS sufferers barely show symptoms while others become mortally ill—or within the viruses themselves. Even though doctors are now almost certain that the novel coronavirus first discovered by the University of Hong Kong (HKU) is the primary cause of SARS, the total number of isolated coronaviruses in patients is still quite low. "So it's difficult to compare in terms of genomic arrangement whether those with a high transmissibility actually have some changes in genetic material," says Dr. K.Y. Yuen, a microbiologist at HKU.

Scientists believe that the likelihood of contracting SARS is partially linked to the amount of virus a person is exposed to, so superspreaders may be shedding a larger number of viruses than ordinary patients. Infectees whose immune systems have already been compromised by a previous illness—the suspected Amoy Gardens superspreader, for instance, had a pre-existing kidney condition—might become viral factories, pumping out huge viral loads. Superspreaders might also be cross-infected with the coronavirus and one of the other pathogens that have been found in some SARS patients—the paramyxovirus or the respiratory-transmitted chlamydia pneumonia bacteria—which could somehow enhance both the disease's transmissibility and virulence.

Or it could be bad luck. Both suspected superspreaders in Hong Kong seem to have been aided by unpredictable environmental factors. The infector at the Prince of Wales Hospital was being treated with a nebulizer, an inhaler that spread his virus-laden respiratory droplets throughout the ward, turning him into a human biological weapon. At Amoy Gardens, faulty plumbing may have enabled virus-contaminated sewage to spread to individual apartments, leading to the worrying possibility that SARS could be transmitted by water.

The mysteries of SARS are multiplying as fast as the disease itself. "From a clinical point of view, there are no clear indicators for us to recognize who will be the ones who spread SARS more effectively than others," says Dr. Ling Ai Ee, a virologist and chief of Singapore's SARS investigation team. With the disease out of the bottle in Asia, hope will rest in the possibility of a vaccine. Thanks to an unprecedented global effort, scientists in the U.S. believe a vaccine could be ready in a little more than a year—provided the new coronavirus doesn't turn out to be as mutable, for example, as HIV. For now, though, the season of SARS shows no signs of abating.

• BRYAN WALSH / HONG KONG
• The latest SARS science