It took only a few dry coughs to spread a mysterious respiratory illness to clusters of health workers in Asia and to kill the World Health Organization doctor who first identified it. And it took only a few airplane passengers for the illness to reach 15 countries in Asia, North America and Europe.

Now, one person has apparently spread the illness to scores of residents of an apartment complex in Hong Kong, and the government has put the entire complex in isolation and closed all schools. In Toronto, two hospitals have been closed to new patients.

The patterns of transmission raise the possibility that the illness, known as SARS for severe acute respiratory syndrome, can be spread through the air or contaminated objects as well as close face-to-face contact.

The events justify the bold alarm that the World Health Organization set off on March 15 when it declared SARS ''a worldwide health threat,'' at a time when cases and deaths were few in number. The organization then hastily created a network of 11 infectious- disease laboratories in nine countries to track down the cause of SARS. Officials at W.H.O., a United Nations agency, cannot recall the last time the agency has issued a global alert for an acute outbreak of a disease. The agency has long had networks of laboratories for influenza and other diseases, but such networks have rarely been pressed into emergency service.

In less than two weeks, with an alacrity and a degree of cooperation seldom seen in science, the laboratories identified two previously unknown viruses as the leading suspected agents. (They belong to the coronavirus and paramyxoviridae families.) But because of the danger of the illness, W.H.O. is restricting research on it to its network. All laboratories in the network operate at the second-highest hazard level, known as P-3, reserved for all but the most deadly pathogens.

Far from being the last link in the discovery process, identification of a new virus in a laboratory is only the first of many steps needed to prove that a suspect virus actually causes a disease. It is likely to take weeks more to determine which virus, singly or in combination, actually causes SARS. In that time, the leading suspects may be displaced by other candidates.

For now, the leading suspect is a coronavirus, though proof is far from certain. The family takes its name from the crown of spikes that surround the spherical virus. Known coronaviruses cause the common cold and are suspected of causing diarrheal and other intestinal illness in humans. Though pesky, the ailments are rarely fatal. But in animals, coronaviruses can cause devastating illness among cats, dogs, chickens, pigs and cattle. Hardy coronaviruses can survive outside the body for up to three hours, allowing a contaminated object to spread infection. The suspect coronavirus has not been identified previously, and for now it is hard to know how deadly it will become.

''Many people have been very blasé about this outbreak, but they are coming to realize it is more important than just any old common cold or some stomach bug that you might pick up on an overseas trip,'' said Dr. Mark W. A. P. Salter, an infectious disease specialist and a leader of the SARS investigation.

Scientists have gained a significant, though rudimentary, knowledge of SARS in a short time. For example, they know the hallmarks of the disease, which are a fever of 100.4 degrees or higher, chills, muscle aches and dry cough. They also know the average incubation period -- the time it takes from exposure to symptoms -- is about 4 days, ranging from 2 to 10. Scientists also know that no drug is effective against SARS. Its spread can be prevented by having health workers wear masks, gowns, gloves and goggles, and by isolating patients.

But many answers are needed to determine the most effective measures to control SARS. These are among the questions: *Do affected people start dispersing, or shedding, virus before symptoms develop?

*For how long after symptoms disappear can infected people shed the virus?

*Does the amount of virus shed correlate with the severity and duration of illness?

*In what tissues and body fluids can the virus be found, and at what stage of illness?

*Can the virus be transmitted through urine and feces as well as respiratory droplets?

*Does the virus survive long enough outside the body that it can be spread by touching contaminated objects like an elevator button or a piece of paper?

*Are there people who can disperse the virus even if they have no symptoms?

*What is the source of the virus? Is it an animal virus that jumped species?

*Are two or more viruses acting in concert to cause some SARS patients to become sicker than others? If so, which viruses?

The answers would provide a more solid base for making crucial decisions, like when it is safe to discharge SARS patients from a hospital and allow them to return to work or school. The current recommendation is the longest incubation period, 10 days after the last symptoms.

The answers would also affect another crucial decision, when to declare that an outbreak has ended. An epidemiologic rule of thumb is twice the longest incubation period -- 20 days for SARS. But because patients can shed viruses after fully recovering from some other infections, health officials must learn whether affected SARS people can still transmit the virus after symptoms disappear.

To prove that a virus identified in the laboratory causes SARS, scientists must develop diagnostic tests to determine how often it was present among patients in the outbreak and at what stage of the illness it appeared. Those steps require collecting and testing specimens from patients in various stages of the disease and from their contacts, and will further strain the taxed network, said Dr. Klaus Stöhr, the scientific director of the SARS investigation.

On March 17, W.H.O. set off a frenzy of activity by creating the network, which involves hundreds of scientists, after its officials, including Dr. David L. Heymann, the executive director in charge of communicable diseases, became deeply concerned about a small number of outbreaks in Hong Kong and Hanoi.

The first flare went up in February, after a rare avian strain of influenza killed one person and sickened another in Hong Kong. The same strain had infected 18 people in 1997, killing 6, when health officials feared it might set off a pandemic of influenza similar to the one that occurred in 1918-19.

The second flare came in Hong Kong on March 13, with the death of an American man who had been transferred from a hospital in Hanoi. The man had been admitted on Feb. 26 for breathing trouble. Soon, a large number of workers in the affected hospitals in Hanoi and Hong Kong developed similar symptoms.

Then, W.H.O. epidemiologists began to wonder whether the 305 cases of a respiratory illness (5 of them fatal) that Chinese officials had reported in Guangdong province from November to February might have been related to the Hong Kong case.

''Everyone was thinking, Perhaps we are looking at a new type of influenza virus,'' Dr. Salter said.

But tests in a Japanese laboratory that received specimens taken from the ill American man excluded many infectious agents, including all known strains of influenza.

The W.H.O. laboratory network includes some that were situated near the affected hospitals and others, continents away, with experience in working with dangerous viruses. The organization also set up a secure Web site for members of the network to discuss their findings in regular teleconferences.

In the first of a series of telephone and Web reports, on March 19, laboratories in Germany and Hong Kong reported finding evidence of a member of the paramyxoviridae family on electron microphotographs of specimens from the small but growing number of patients who were becoming ill with SARS.

A day or two later, two network members, the Centers for Disease Control and Prevention in Atlanta and the Princess Margaret Hospital in Hong Kong, reported that their scientists had seen electron microscopic evidence of a coronavirus. Around March 20, Canadian scientists reported evidence in a few patients hospitalized in Toronto suggesting that the cause was a second member of the paramyxovirus family -- a metapneumovirus that differs from the paramyxovirus detected earlier. But because this is the peak season for metapneumovirus infections, network scientists have debated the significance of the finding and have since dropped that virus to a distant third on the list of SARS suspects.

Now laboratories in at least eight countries have isolated the coronavirus from a number of patients, including two in the United States, said Dr. James M. Hughes, an official of the C.D.C.

Scientists can use a number of steps to try to identify a virus. A specimen can be put in a glass beaker lined with cells to see whether it damages or kills the cells. Because viruses are finicky, scientists must use different types of cells to detect various viruses.

Another step is to infuse the beakers with the serum part of the blood from patients as they recover from an infections. The aim is to determine whether the serum, which may contain antibodies to the virus, prevents the death of cells. Last week, several network scientists found such blocking, indicating that some SARS patients had formed the specific antibodies against the suspect coronavirus.

Scientists have also used a technique known as polymerase chain reaction, which can identify as few as one or two infectious agents in a sample to identify the suspect coronavirus in urine and feces. But they have yet to determine whether what they detected was a virus that can cause infection or merely harmless remnants.

To help classify a virus, scientists need to know its basic structure, or molecular sequence. The first bits of sequence information were distributed to members of the network through the secure Web site. So far, the suspect coronavirus's sequence most closely resembles that of the murine hepatitis virus, which causes a disease similar to multiple sclerosis in mice. Mildly abnormal liver function tests have been reported among SARS patients, although SARS does not seem to cause full-blown liver disease in humans. The full sequence is expected within two weeks, said Dr. Julie L. Gerberding, the C.D.C. director.

Last week, scientists developed the first two diagnostic tests for the coronavirus -- one an antibody test, the other an immunofluorescent test. But scientists must still test a large number of patients to determine such scientifically important information as how often the tests fail to detect cases and how often they register positive when somebody does not have SARS.

Testing will allow scientists to determine how many household and other contacts of patients were infected but did not develop symptoms.

Although scientists at the network are swamped with work, W.H.O. does not want to enlarge it for now. ''We have not expanded the network,'' Dr. Salter said, ''because we are looking at a very dangerous virus and we don't want to encourage laboratories elsewhere to start working with it if they do not have the appropriate containment facility.''

Photo: Dr. David L. Heymann of the World Health Organization, which set up a network to investigate SARS. (The Associated Press)(pg. F1) Chart: ''From One Hotel Guest, Many Infections'' As of March 26, these 249 cases of sudden acute respiratory syndrome, or SARS, had been traced to one man. Other cases have surfaced since, but officials can't be sure which ones are related to the same man. IN VIETNAM One hotel guest travels to Hanoi; 37 medical personnel are infected. He returns to Hong Kong. Four die. BANGKOK In addition, a World Health Organization doctor traveling from Vietnam becomes ill. He dies. IN SINGAPORE Three hotel guests infect 34 health-care workers... ...and 37 others. Two die. A physician in Singapore travels to New York City. The doctor attends a conference in New York. He leaves for Singapore but is detained and hospitalized in GERMANY. Two relatives traveling with him become infected. 1. A Chinese medical professor who had been treating SARS patients in Guangdong becomes infected. IN HONG KONG 108 people are infected by the professor and two guests. HOTEL METROPOLE The professor stays overnight Feb. 21. A total of 12 other guests become infected. A guest infects 99 medical personnel. A guest infects three medical personnel. This guest dies in Hong Kong. The professor infects two relatives and four medical personnel. He dies. CANADA One of two hotel guests infects four in her family; she and one member of her family die. They infect 12 others; one dies. UNITED STATES -- Three infected hotel guests return. Two are a married couple who stayed at the hotel after the professor but during the same time as three of the other ill guests. (Sources: Centers for Disease Control and Prevention; Dr. Stephen M. Ostroff, C.D.C.)(pg. F6)