lmost from the moment the new Asian respiratory disease was first identified last month, scientists — and many ordinary citizens in chat rooms on the Internet — wondered whether it might have been the work of terrorists. Now medical and military experts say the answer is almost certainly no.

"As a scientist, you never say never," said Dr. Steven M. Block, a biologist and germ-weapons expert at Stanford University. "But every indicator I'm aware of points to a natural outbreak."

While the cause of severe acute respiratory syndrome, or SARS, has not been pinned down, the leading suspect is a virus from the coronavirus family, and the experts say there are several reasons to think its origins are natural: its birthplace in a region of China long known as an incubator for new kinds of influenza, its relatively low lethality and the individual biology of coronaviruses.

But, they add, that biology makes it menacing in a different way.

Coronaviruses have a singular talent for recombination — for absorbing bits of stray genetic material. One day, virologists warn, that tendency might suddenly turn a benign coronavirus into a deadly one.

"It has the highest frequency of recombination that we know of for any positive-strand RNA virus," said Dr. Susan C. Baker, a virologist at Loyola University in Chicago. "With high-frequency recombination, you always have potential for a new virus to emerge." Now, she added, "it looks like it's happening."

Known coronaviruses, in addition to causing the common cold, are suspected of causing diarrheal and other intestinal illnesses in humans. Though bothersome, the ailments are rarely fatal. But coronaviruses have caused major illnesses among cats, dogs, chickens, pigs and cattle.

Experts say the new human coronavirus, if it causes SARS, probably arose when it managed to incorporate similar but foreign RNA, which, like DNA, can make up the genome or genetic code of microorganisms. Such alien RNA would make it a kind of natural hybrid.

Human coronaviruses, said Dr. Mark R. Denison, an expert at Vanderbilt University, are like the mild-mannered next-door neighbor with a proclivity for doing the unexpected. "It's always the quiet ones you worry about," he said.

Federal experts echo the university experts. "I see no reason to believe this is anything other than the emergence of a natural disease," said Dr. Peter B. Jahrling, a virologist at the Army's Medical Research Institute of Infectious Diseases.

Coronaviruses, he added, "are ubiquitous and are relatively promiscuous" in their ability to infect different species. Infection of a single host with two different coronaviruses can easily lead to recombination and the emergence of new forms, he said, and "that's probably what happened here."

Guangdong Province in southern China, where the illness is believed to have emerged late last year, has dense concentrations of domestic waterfowl in close proximity to pigs and people. Experts say those are ideal conditions for transferring diseases among different species and for the emergence of a new strain of flu virtually every year. "It's no surprise that other viruses can take advantage of similar mechanisms," said Dr. Block of Stanford.

Dr. Richard H. Ebright, a microbiologist at Rutgers University who studies germ weapons, agreed that the Chinese origin of SARS argued against its being a weapon. "If it had first been detected in New York or Washington, or Kuwait City or Tel Aviv," he said, "then I'd think there'd be strong reason for concern."

A military expert who disagrees is Dr. Ken Alibek, a former top Soviet germ warfare official now at George Mason University. In his book "Biohazard" (Random House, 1999), Dr. Alibek said China had developed biological weapons and had once suffered an accident at a secret germ plant, setting off two epidemics.

SARS, Dr. Alibek said in an interview, might have originated from a similar accident. "It's a very unusual outbreak," Dr. Alibek said. "It's hard to say whether it's deliberate or natural." He added that he knew of no Chinese germ-weapons plants in Guangdong.

Several biologists cited the low lethality of SARS — it kills somewhere from 3 to 5 percent of its victims — as evidence of its natural origin. Dr. Block of Stanford noted that the germs and viruses that cause diseases like anthrax and smallpox typically have mortality rates of 25 to 95 percent.

"It's bad," Dr. Ebright of Rutgers said of SARS. "But 3 percent is not the Andromeda strain."

The experts agreed that pinpointing the disease's place of origin in Guangdong could help settle any doubts about whether SARS is deliberate or natural. In China, a team of scientists from the World Health Organization is trying to discover how the illness started and spread.

Mapping the genes of the suspect coronavirus will also shed light on its origin, biologists said. Genes from a highly dissimilar organism would point to human genetic engineering. "If they put in genes from Ebola," said Dr. Block, referring to a dangerous germ from Africa, "that would be a dead giveaway."

The Centers for Disease Control and Prevention in Atlanta, which is leading the American inquiry of SARS, said last week that the mapping, known as sequencing, should be completed by the end of this week or the beginning of the next one.

"The sequencing information will show us right away where it came from," said Dr. Baker of Loyola. "I think it's highly unlikely that anybody could have created this virus."