A deadly, mysterious respiratory illness spread largely among health care workers in Asia could be a new strain of flu or even an exotic virus passed from animals to people, a health official said Sunday.

Probably the most feared by health experts, however, would be a new and deadly strain of flu.

The illness, which carries flu-like symptoms, has killed nine people – seven in Asia and two in North America. Its rapid spread in southeast Asia in recent weeks caused a rare worldwide health alert to be issued on Saturday.

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.

“Certainly influenza is on the minds of many people,” said Dr. David Heymann, communicable diseases chief for the World Health Organization.

Lab tests have ruled out some varieties of flu as well as some viruses that cause hemorrhagic fever. However, many other possibilities remain, Heymann said.

Those include “a new strain of influenza” or such exotic diseases as the closely related Hendra and Nipah viruses – both newly recognized, causing flu-like symptoms and capable of being spread from animals to people.

“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,” the year a worldwide flu epidemic killed at least 20 million people.

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 (news – web sites) in Atlanta.

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.

Heymann 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 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 flu-like 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.

The WHO estimates that perhaps 500 people in all have been sickened if an earlier outbreak that peaked last month in Guangdong province 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.

Tests so far have ruled out the H5N1 bird flu, which has popped up occasionally in China and which many fear could be catastrophic if it spread widely among humans.

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.

Also on Sunday, the WHO released a report from the China Ministry of Health on the Guangdong outbreak, which said “the epidemic situation has been controlled and the patients are being cured one by one.”

In that outbreak, the Chinese said, most victims were young adults, and the disease apparently was spread similarly to SARS. The outbreak peaked between Feb. 3 and 14 in Guangzhou City and has since decreased markedly.

The Chinese said 7 percent of patients required breathing tubes, but most eventually got better, especially if they were not also infected with bacteria. The disease seemed to weaken as it passed from person to person.