Experts say there is no evidence to suggest the illness is related to bioterrorism.
Health experts searching for the cause of a frightening outbreak of a deadly flulike illness in Asia say the culprit is probably a virus, and they are encouraged that some victims appear to be getting better.
More than 150 people have fallen ill, mostly in Hong Kong and Vietnam, over the past three weeks. And experts suspect that an additional 300 people in China's Guangdong province had the same disease beginning in mid-November.
While experts are unsure precisely what is causing the outbreak, several say their biggest fear is that it is a new and lethal form of influenza.
"If it really is the flu, it could be we have a new organism that could cause a pandemic," said Dr. R. Bradley Sack, director of Johns Hopkins' international travel clinic. "People immediately start thinking of 1917," when a worldwide flu epidemic began that killed at least 20 million people.
So far, the disease has killed nine people -- seven in Asia and two in North America. Its rapid spread, and the discovery of two clusters in Canada, caused a rare worldwide health alert to be issued Saturday.
Health officials in China said today that the disease that infected about 300 people and killed five in Guangdong province "seems amenable to treatment," although they stressed there is still no link to cases in other countries.
Asian airports were screening passengers for flulike symptoms, in the hope of stopping the spread of the disease. Some fearful passengers wore surgical masks or covered their faces to ward off infection.
Experts discounted the possibility that terrorism is the source and believe it almost certainly is a contagious infection that spreads most easily from victims to their doctors, nurses and families through coughing, sneezing and other contact with nasal fluids.
"Nothing about that pattern suggests bioterrorism," said Dr. Julie Gerberding, head of the U.S. Centers for Disease Control and Prevention in Atlanta.
Health officials say it may be several more days before they are able to identify the disease. However, they said several of its features suggest it is caused by a virus, which can often be difficult to pinpoint quickly using standard lab tests.
The illness is being called "severe acute respiratory syndrome," or SARS. The incubation period appears to be three to seven days. It often begins with a high fever and other flulike symptoms, such as headache and sore throat. Victims typically develop coughs, pneumonia, shortness of breath and other breathing difficulties. Death results from respiratory failure.
The World Health Organization has been aware of the outbreak for about three weeks but issued its global alert this weekend because of concern that the illness would spread to North America and Europe.
Officials said they are encouraged that some recent victims seem to be recovering, although they are unsure whether that is because of the many antibiotic and antiviral drugs they have been given or simply the natural course of the disease.
Dr. David Heymann, WHO's communicable diseases chief, said three or four patients had stabilized enough to be moved out of intensive care Sunday in Hanoi, Vietnam, although all still had breathing problems.
The World Health Organization estimates that perhaps 500 people in all have been sickened if the earlier outbreak in China turns out to be part of the same disease, as they suspect it is.
Ninety percent of the most recent cases have been in health care workers.
The CDC prepared cards that were being given to travelers arriving from Hanoi, Hong Kong or Guangdong province in China, warning they may have been exposed. It recommended they see a doctor if they get a fever accompanied by a cough or difficulty breathing over the next week.
Investigators suspect a virus is involved, because victims do not seem to respond well to standard antibiotics, which kill only bacteria, and because their white blood counts drop. That typically happens with viral infections but not bacterial ones.
Few drugs exist for treating viral diseases and often they must run their course until brought under control by the body's natural immune defenses.
No cases have been confirmed in the United States, but Gerberding said the CDC is checking out a few calls. The North American fatalities were a woman and her grown son who died in Toronto after visiting Hong Kong.
A 32-year-old physician from Singapore suspected of having the disease was taken off an airliner during a stopover in Frankfurt, Germany, on Saturday after being in New York City for a medical conference. He was held in quarantine, along with his mother, who had a fever, and his wife, who remained healthy.
However, on Sunday, the man's physician, Dr. Hanns-Reinhardt Brodt, said he was uncertain the case was SARS; he was treating him for ordinary pneumonia.
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