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Still, the large number of cases in Hong Kong's Metropole Hotel and Amoy Gardens apartments suggests to Thompson that "there is something going on, a form of transmission that we don't understand." Researchers are concerned that at least some victims--the Chinese doctor at the Metropole and someone in Amoy Gardens, perhaps--are so-called superspreaders. For unknown reasons, some people pass the virus along much more easily than most, perhaps because of the way they cough or the types of contact they have with others or because they carry an unusually heavy load of the virus. Says Ostroff: "We have not seen superspreading in the U.S., but it's a theory that has been advanced in both Canada and Asia." Kwan Sui-chu, the woman who carried SARS from Hong Kong to Canada, is believed to have infected as many as 155 people.
Paradoxically, the existence of superspreaders could be good news: if just a few victims are responsible for the bulk of the outbreak, the disease should be easier to control with aggressive quarantine measures. Some doctors have also postulated that as the virus radiates from the initial patient, it becomes less virulent, meaning that those who come down with second- or third-generation infections may be in less danger. That could explain why in the U.S. nobody has died and only one case has even turned out to be life threatening. Says Dr. Joseph Sung, chief of service at Prince of Wales Hospital: "I'm quite convinced that some people might have contracted the infection but not the disease. Some may develop mild symptoms, like a little bit of cough and no fever; some may just feel a little tired for a day or two."
Mild cases would also act like natural vaccines, conferring immunity on the patients. When 70% or so of a population has immunity--what epidemiologists call herd immunity--a virus is considered burned out: it can't spread further because there is almost no one left to infect. In the meantime, hospitals are trying to prevent the disease from taking root by isolating patients and religiously using gowns, masks, goggles and gloves--techniques that so far have proved extremely effective at preventing transmission.
Still, although the number of new cases in such hot spots as Singapore and Hong Kong finally began to drop last week, nobody is prepared to say that this outbreak is even close to being over, especially considering that many residents of Hong Kong's Amoy Gardens apartments fled before officials could arrive to take them into quarantine. "I don't want to give anybody a false expectation that this is under control," says Ostroff. "This is still a very fluid situation."
So while public-health officials stay on high alert for any new cases, researchers are trying to erase any lingering uncertainty about the ultimate cause of SARS. Several kinds of diagnostic tests are nearly ready. A vaccine against the coronavirus is already under development. And epidemiologists in Asia, Europe and North America are piecing together the natural history of SARS. That last effort should be made a lot easier with the Chinese government's decision last week to finally let a WHO team into Guangdong. Medical detectives may have already found the very first recorded victim--patient zero--a man in the city of Foshan who passed the virus on to four people before he recovered.