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The pieces fell into place fairly rapidly. Of the first 45 people to contract SARS in Hong Kong, most had contact with employees of the Prince of Wales Hospital or family members of patients. Those people in turn had contact with a smaller circle who treated or visited a 26-year-old male patient, originally diagnosed with a nonspecific fever. That man, Hong Kong authorities determined, had visited a friend on the ninth floor of the Metropole Hotel in the Hong Kong district of Kowloon in February.
They also discovered that six other people who had stayed on the same floor of the hotel between Feb. 12 and March 2 had SARS too. One of them was a 64-year-old doctor from Guangzhou, the capital of Guangdong. Could he be the source of all the Hong Kong cases? "We believe this hypothesis is justified," says Dr. Margaret Chan, Hong Kong's director of health. At least one other Guangdong man took SARS with him to Spain, while several Metropole victims fanned out across the world to Singapore, Vietnam and Canada. People infected by them in turn spread the disease even farther afield.
Once they realized what was going on, officials at WHO issued an unprecedented global alert and, along with the CDC and other disease labs, launched an impressively coordinated effort to understand the illness as quickly as possible. By examining the victims' body fluids, pathologists at the University of Hong Kong determined that the probable culprit was a mutated coronavirus, although some victims also seem to be carrying a type of paramyxovirus, a member of the family that causes measles. It could be a helper virus that makes the coronavirus more virulent, or it could be an unrelated coinfection. For now, says WHO's Thompson, "we're proceeding as if we know [the cause is] the coronavirus for certain." Both the CDC and Hong Kong University are developing laboratory diagnostic tests for the virus and its antibodies to replace the much less accurate symptom-based diagnoses being used today.
Medical detectives are also trying to understand exactly how SARS is spread. The illness appears to infect the lower lobes of the lungs (unlike common-cold viruses, which typically attack the throat and nasal passages). That's not a good thing: viruses rising from the lungs tend to be distributed in fine aerosols and thus travel farther than the relatively large droplets expelled in sneezes and bronchial coughs. "The data we've heard certainly don't suggest that aerosol transmission is common," says the CDC's Ostroff, "but we keep an open mind to that possibility."