The World Health Organization yesterday took the unprecedented step of warning travelers to stay away from Hong Kong and China's Guangdong province because an epidemic of a dangerous new lung infection is still spreading mysteriously there.

It was the first time in its 55-year history that the Geneva-based United Nations body has cautioned people not to travel to a specific location because of a disease. WHO, which previously had issued travel advisories only because of war or other conflicts, was hesitant to take the step because it was expected to have serious economic consequences, especially for the financial center of Hong Kong.

Officials said they felt compelled to take the drastic measure because the epidemic remains out of control and is being transmitted in ways that have surprised and stymied scientists. In addition, there is no vaccine to protect people and no sure way to treat victims of the illness, which can be fatal.

"This is the first time that we have recommended people avoid an area," said David L. Heymann, who heads the WHO's communicable diseases program. "And that is, of course, because we do not understand the disease completely, because there's no vaccine and there's no drug."

The disease, known as severe acute respiratory syndrome, or SARS, is believed to have sprung up in the southern province of Guangdong in November and then spread to Hong Kong. There, a number of travelers became infected and carried it to Toronto, Singapore, Vietnam and possibly elsewhere in late February and early March. At least nine additional travelers, tourists or business executives from Beijing, Taiwan and Singapore have returned from Hong Kong with the disease since March 15, when WHO took its first major step to try to contain the epidemic by issuing an unusual worldwide alert.

The disease appears to have been controlled in Hanoi and possibly Singapore and Toronto, but not without stringent measures such as closing schools and quarantining hospitals. Suspected cases have continued to appear around the world, with 2,223 cases now reported in 17 nations. At least 78 people have died. Officials in Brazil yesterday reported that country's first possible case, which would be the first SARS case in Latin America.

Fears have intensified because the disease, which had been thought to spread only through close contact with an infected person, now appears to be transmitted in other, unknown ways. Hong Kong this week evacuated 240 people from a 33-story apartment tower and moved them to quarantine camps because the virus was spreading in their building.

"Transmission does not seem to be only by close contact from person to person," Heymann said. "It appears that there is something in the environment that . . . is serving as a vehicle to transfer the virus from one person to another."

The virus believed to cause SARS does not appear to be spread through the air, he said. Instead, investigators "believe it is something else in the environment. It is possibly an object that people are touching and getting infected from, where there has been a SARS patient who has coughed or possibly a sewage system or a water system or some type of environmental vehicle that takes the virus from a sick person to others."

The virus is believed capable of surviving on an inanimate object for several hours and has also been detected in urine and feces.

"We see clusters of cases where there is one case, for example, living in an apartment building, where other people in that apartment building have been infected," Heymann said.

Hong Kong has been among the hardest hit, with at least 708 people ill and 16 dead.

WHO had previously taken steps short of a travel warning to stem the spread of the disease, recommending that travelers to and from areas where outbreaks are occurring be informed about the disease, and that air travelers be screened for symptoms.

The United States last week warned travelers to avoid Hong Kong, Singapore, Hanoi and all of mainland China. Health officials have been meeting all planes, boats and cargo ships from Asia, removing and isolating sick passengers and distributing cards to other passengers instructing them to look for symptoms.

Other nations have taken even more drastic steps. Thailand, for example, is requiring everyone entering the country from an affected area to wear surgical masks. Malaysia has banned the hiring of workers from affected areas.

The disease typically starts with a fever of 100.4 degrees or higher, a dry cough and other flulike symptoms. At least 80 percent of patients recover, but the rest deteriorate. Health officials estimate that about 4 percent die.

WHO's travel warning came as China finally agreed to allow a team from the agency to go to Guangdong to investigate the epidemic. WHO has been pressing for permission for weeks to search for clues on how the epidemic began and how to end it.

Also yesterday, the Chinese provided more information about the outbreak, reporting that there have been 1,190 cases in six provinces and 46 deaths, the most in the world. One bright spot was that the rate of infection in Guangdong appears to be falling. There were 145 new cases between March 1 and 10, China reported, but only 88 new cases from March 21 to 31.

At least 85 suspected SARS cases have now been reported in 27 U.S. states, including two in Virginia. Most of the patients have been international travelers, but five health care workers and two family members have been infected by travelers.

In Atlanta, the U.S. Centers for Disease Control and Prevention announced yesterday that it planned to provide state health officials two tests it developed to detect signs of the suspected virus in blood and tissue samples of possible SARS victims.

The test detects antibodies produced in response to the suspected cause, a so-called coronavirus. Coronaviruses usually cause nothing more serious than the common cold. SARS appears to be caused by a previously unknown coronavirus, perhaps a human virus that mutated or an animal virus that jumped species.

CDC Director Julie L. Gerberding cautioned that the virus is not the proven cause of the disease, and that a positive test result may not necessarily mean someone has SARS.

But "if a patient has an antibody response to the new coronavirus, it may be very strong evidence of an association with SARS," she said. "It's a useful tool. What we are seeing so far [is that] people who have the strongest epidemiological link to SARS are turning out to have positive tests."

A British traveler in Hong Kong wears a mask to protect herself against severe acute respiratory syndrome, or SARS. The World Health Organization warned travelers yesterday to stay out of Hong Kong and China's Guangdong province.