y now, who hasn't heard of SARS, or severe acute respiratory syndrome?

The disease is thought to have originated in southern China in November, but to have been covered up for months by the Chinese government.

As of Friday, according to the World Health Organization, 2,353 people in 16 countries were reported to have the disease, and at least 84 had died — 3.5 percent — mostly from pneumonia. Fears of the disease have led American health officials to discourage travel to Hong Kong, Vietnam, Singapore and mainland China — causing havoc among tourists and seriously affecting the global economy.

In the United States, there were at least 115 suspected cases, 109 in people who had traveled to affected parts of Asia, and the rest in people who were exposed to those travelers. As of Friday, no one had died. deaths. Health workers are now meeting airplanes arriving in the United States from affected countries, and handing out cards that urge passengers to be on the lookout for SARS symptoms like cough and fever. Also on Friday, President Bush signed an executive order allowing the Health and Human Services secretary to order the forced quarantine of infected individuals.

SARS is the latest in a series of "emerging infectious diseases," a list of previously unknown illnesses that runs from Lyme disease, to Legionnaire's disease, hantaviral lung disease, West Nile virus and AIDS.

It is too soon to say how far SARS will spread, but experts say it is an urgent public health threat. One reason is that unlike the other emerging diseases, SARS is spread through the air, by coughing and sneezing. In addition, though some people who died of SARS were elderly or suffering from underlying illnesses that made them more vulnerable, others were relatively young and perfectly healthy, and doctors do not understand why they succumbed.

"A 3.5 percent death rate in the current era, that's notable," said Dr. William Schaffner, director of the division of infectious diseases at Vanderbilt University. "This is a new respiratory illness, undefined, and it has, under certain circumstances, a propensity to spread rapidly, and it had an immediate international component in terms of spread. In that sense it has overtones of new influenza outbreaks that have similar kinds of characteristics, and there it's pretty high up on the urgency list."

One thing that is not clear is whether the death rate really is 3.5 percent. The real rate is uncertain because no one knows how many people are infected; some may have minor symptoms or none at all. If so, the death rate may actually be much lower.

Dr. Scott Hammer, chief of the division of infectious diseases at Columbia Presbyterian Medical Center in Manhattan, said it was worrisome that the disease could spread quickly through the air via coughing and sneezing.

"Whenever you have an illness that does that it naturally creates anxiety in the public and public health authorities, particularly if there is a 3 to 4 percent fatality rate," he said.

BUT so far, experts say, most Americans do not have to worry about catching the disease. "If you haven't traveled to an affected area or been exposed to an ill patient, there's no evidence you're at risk at all," said Dr. James Hughes, director of the National Center for Infectious Diseases at the Centers for Disease Control and Prevention. At the same time, Dr. Hughes said, the possibility of a worldwide epidemic cannot be ruled out. The speed and volume of international travel mean people can unknowingly carry diseases all over the world even before the incubation period is up.

As to where the virus came from — assuming it is, as suspected, a new type of coronavirus (a virus that is one of the chief causes of the common cold) — experts say there are several possibilities. One is that an earlier coronavirus that did not cause severe illness in people underwent a genetic shift that made it more virulent. Another is that a coronavirus changed for the worse by trading genetic material with another virus.

It is also possible that an animal virus "jumped" into humans. Such jumps are known to occur, occasionally with dire results. A virus and its usual host may have adapted to each other to coexist. But with a new host, no truce has been made.

"You haven't had co-evolution of the virus and the host over time," said Dr. Joshua Lederberg, a microbiologist and a Nobel laureate in medicine.

But a virus that kills its host too quickly may wind up with no place else to go. "Virulence is kind of a side effect of the aggression a virus needs to get over the host's defenses and make a foothold," Dr. Lederberg said. "The virus must earn a living, but it does better to domesticate the host than to kill it."

A fourth possibility, which scientists consider unlikely, is bioterrorism: a new virus might have been genetically engineered in a laboratory, and might then have escaped or been released on purpose.

"There is no evidence for this, but we have to think about it in this day and age," Dr. Schaffner said.

It is far too early to tell how important SARS will be in the future, experts said. Dr. Hammer noted that many respiratory illnesses, like the flu, tend to come and go seasonally, and scientists do not know why.

"Is this here to stay?" he asked. "Will it be a continuous threat, a seasonal threat? How will it come into the broad spectrum of respiratory illnesses and infections for the future? Even if it ebbs, we won't know about this until next year."