SARS is claiming fewer victims. On June 12 hard-hit Hong Kong experienced is first day without new SARS cases or deaths, and the World Health Organization has lifted its travel advisory on some parts of China. But it's too early to assume the disease will live on only in medical-school case histories. In Toronto, where officials thought SARS was all but eradicated in late April, doctors are dealing with a second outbreak in which more than 100 infections have been traced to the fourth floor of Toronto's North York General Hospital. Last week, officials were also investigating whether a U.S. man who visited a home for the elderly in Toronto in mid-May could have contracted SARS from a patient who was not showing symptoms at the time. Doctors say the chance of asymptomatic transmission is slim, but the possibility is worrying. "That changes the picture considerably because the whole basis for believing that you can put SARS back in the box is based on the fact that you can detect all the people who may be spreading it," says Dr. Stephen Corber, a disease-prevention and -control manager for the WHO in Washington, D.C.
Development of a fast, reliable diagnostic test is still many months away, so doctors are forced to diagnose SARS cases on the basis of symptoms, such as high fever, that can be easily confused with other ailments. Some patients—particularly elderly ones, who often suffer a host of maladies—can go undetected. Toronto shows the damage that can be done with a single slipup. "It really tells you how unforgiving this disease is," says Dr. Allison McGeer, a microbiologist who heads Mount Sinai Hospital's infection-control department in Toronto.
Even if asymptomatic infectees aren't spreading SARS in large numbers, their presence makes a resurgence of the disease more likely, because they provide a human reservoir in which the coronavirus that causes SARS can thrive and mutate. Equally alarming, the list of animal hosts also increased last week as researchers in China's Guangdong province, believed to be the origin of the epidemic, reported that a wide variety of wild animals—in addition to civet cats and raccoon dogs—now seem to carry a close version of the virus, which could jump to humans.
Even if health officials have successfully bottled up this outbreak, the disease could return next year—possibly in a more infectious form. Initial hopes that the virus would weaken as it passed through the human population proved false. Worse, mutations in the virus seem to accumulate and spread rapidly, which increases the likelihood that a nasty new variant could appear in the future—possibly next fall or winter, when environmental conditions could make SARS more contagious. (Some scientists theorize that SARS, like similar viral infections that produce colds, could settle into a seasonal pattern of infection.) "What we have to realize is that SARS may be with us to stay," says Dr. Leong Che-hung, chairman of the Hong Kong Hospital Authority. Unless a cure is found, the best hope is that SARS eventually loses its punch.