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U.S. health officials say that the mysterious respiratory illness that has sickened hundreds of people around the world might be caused by a different virus from the one first suspected. In recent days the World Health Organization said its team of 11 laboratories worldwide had suggested that the cause of the illness, known as SARS, for severe acute respiratory syndrome, might be a previously unknown member of the paramyxoviridae family of viruses. But Monday, the U.S. Centers for Disease Control and Prevention, a member of the World Health Organization team, announced it had found very strong evidence that 'a previously unrecognized virus from the coronavirus family is the leading hypothesis' for the cause of SARS. Among other illnesses, coronaviruses cause the common cold. 'This virus could be, if not the entire cause of SARS, at least contributing to SARS,' Dr. Julie Gerberding, director of the centers, said at a news conference. But she cautioned that it was 'very premature to assign a cause.' Dr. Klaus Stoehr, a virologist who heads the investigation of SARS for the World Health Organization, said the cause could be a paramyxoviridae virus alone, a coronavirus alone or a co-infection of both. 'There is no clear- cut evidence to rule out either virus, so we cannot drop any hypothesis now,' Stoehr said in an interview. 'The investigation has become relatively complex.' Identifying a specific cause is critical because it would allow health officials to better control outbreaks and possibly find a specific therapy

The new figures are being released just days after a World Health Organization team arrived in China to help investigate this country's epidemic of the mystery pneumonia, which goes by the name SARS, for severe acute respiratory syndrome. A clearer picture of the course of SARS emerged on Wednesday after 80 doctors who have treated cases in 13 countries held a teleconference moderated by Dr. [Salter]. In a second group, from 10 to 20 percent of patients develop increasing difficulty in breathing. Such patients usually required breathing assistance with a mechanical ventilator, and many have had to stay on ventilators for a long time. Most SARS deaths have involved patients in this group

In Canada, health officials asked that the thousands of people in Toronto who have been in the Scarborough Grace Hospital since March 16 isolate themselves at home for 10 days from the time of their visit. Two people at the hospital have died from SARS. Premier Ernie Eves of Ontario declared SARS a provincial emergency yesterday. In Hong Kong, top officials yesterday ordered more than 1,000 family members of SARS patients to stay home except for visits to government clinics to check for the disease. Those who violate the quarantine face fines of up to $640 and up to six months in jail. In addition, the government closed all primary and secondary schools for nine days as the number of suspected cases climbed by 51, to 367. Singapore decided on Monday to close all schools and restrict SARS patients to their homes. Based on the spread and course of the illness, health officials have repeatedly said that it takes close contact to transmit the SARS agent, presumably a virus, from an ill person to other people. Individuals most likely to transmit SARS are those who are quite sick and have a fever and cough. Most cases have involved health-care workers who have cared for patients, their family members and friends

'This is new. We don't know a lot about it, and we have a lot of questions about the overall spread' of SARS and 'the possibility of airborne transmission,' she said. 'We are very concerned about the speed' with which SARS has spread in Hong Kong. There's epidemiologic evidence that, for unknown reasons, some patients can transmit SARS to other people much more efficiently than other SARS patients can. [Julie Gerberding] cited one patient in Hanoi, Vietnam, who spread SARS to 56 percent of the health workers with whom the patient came in contact

Although Canada has reported 130 probable and suspected SARS cases, including six deaths, and Ontario on Monday suspended all nonurgent surgery and outpatient clinics in all of its 215 hospitals, the centers said it ''is not advising against travel to or from Canada because there is not evidence of widespread community transmission'' of SARS. From the start, World Health Organization officials have said that SARS generally requires sustained close face-to-face contact. But now its officials are investigating whether SARS can be spread through the air, water, sewage and contaminated objects. Until these and other issues can be settled scientifically -- a process that may take weeks -- health officials can only give advice and act on reasonable assumptions. Doctors do not yet know how long a patient can disperse the virus, even if symptoms have disappeared. In most cases, doctors are making only educated guesses as to when it is safe to let SARS patients leave the hospital. The proper use of barrier nursing techniques like masks, gowns, gloves and goggles have stopped the spread of SARS to the health workers caring for patients, although centers officials say they do not know if such measures are 100 percent effective

The apartment complex outbreak has led World Health Organization officials to focus more attention on the possibility that the illness, known as SARS, for severe acute respiratory syndrome, could be spread in a way other than face-to-face contact. The spread would require intimate contact with the SARS agent. Among the possibilities: sewage, contaminated water and objects like doorknobs and elevator buttons. Dr. [Yeoh Eng] said this evening that the government was investigating whether the virus might have been spread by leaking sewage. Laboratories here have confirmed that the virus is present in fecal matter, he added. But Dr. [David L. Heymann], the World Health Organization official, said that scientists do not know whether the virus detected in feces is a form that can transmit SARS. Health workers gathered at the entrances of Block E of Amoy Gardens in Hong Kong yesterday. (Associated Press); Some residents of Amoy Gardens, a Hong Kong housing complex, left yesterday when a 10-day quarantine was imposed on one of the towers in the development and the number of cases of SARS continued to rise. (Reuters)

The director of the U.S. Centers for Disease Control and Prevention has warned that a mysterious respiratory disease, which has infected hundreds of people worldwide, could be spread very efficiently through close contact, and expressed deep concern that it might also be spread through the air or on contaminated objects. The warning from the disease agency director, Dr. Julie Gerberding, came Saturday as officials continued to monitor developments in Hong Kong where the disease, known as SARS, for severe acute respiratory syndrome, may have sickened 78 people in one apartment complex. 'The potential for infecting large numbers of people is very great,' Gerberding said. 'We may be in the very early stages of a much larger' epidemic of a disease for which there is no specific treatment beyond standard supportive nursing and respiratory care, she said. 'This is new, we don't know a lot about it, and we have a lot of questions about the overall spread' of SARS and 'the possibility of airborne transmission,' she said in a conference call from Atlanta with reporters. 'We are very concerned about the speed' with which SARS has spread in Hong Kong, Gerberding added. As of Sunday, 15 countries have reported more than 1,600 SARS cases, including 55 deaths, to the World Health Organization

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 Stohr, the scientific director of the SARS investigation. 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. 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

It took only a few dry coughs to spread a mysterious respiratory illness to clusters of health workers in Asia and to kill Carlo Urbani, 46, 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 at a time when cases and deaths were few in number, on March 15, when it declared SARS 'a worldwide health threat.' The organization then hastily created a network of 11 infectious-disease laboratories in nine countries to track down the cause of SARS. Officials at the World Health Organization, a UN 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, the UN agency 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

Although Canada has reported 130 probable and suspected SARS cases, including six deaths, and Ontario on Monday suspended all non- urgent surgery and outpatient clinics in all of its 215 hospitals, the CDC said it 'is not advising against travel to or from Canada because there is not evidence of widespread community transmission' of SARS. Also, doctors do not yet know how long a patient can disperse the virus, even if symptoms have disappeared. In most cases, doctors are making only educated guesses as to when it is safe to let SARS patients leave the hospital. The proper use of barrier nursing techniques like masks, gowns, gloves and goggles have stopped spread of SARS to the health workers caring for patients, although officials of the CDC say they do not know if such measures are 100 per cent effective. Photo: CANADIAN PRESS; Commissionnaire Salman Noor wears a mask for protection from the SARS virus as he directs traffic at Pearson International Airport in Toronto. Paper bulletins asking travellers with symptoms of SARS to delay their flights were posted at Pearson on Tuesday, as Health Canada escalated efforts to stop the spread of the potentially deadly ailment