Despite recent victories in the battle against SARS, health authorities are warning that the new disease remains a looming global menace that could flare up again in hotspots that appear under control, spread to new, more vulnerable parts of the world or return in a second, more devastating wave of outbreaks.

Because the epidemic is still raging in China, the world's most populous nation, the danger is rising that infected people will spark outbreaks elsewhere, especially in less developed parts of nearby Asia and Africa.

At the same time, epidemics tend to be unpredictable. They often occur in successive, sometimes more deadly waves, and frequently follow a seasonal pattern, especially in the case of respiratory infections.

"I wish we could say we had turned the corner," said David L. Heymann, executive director of the World Health Organization's communicable diseases program. "The issue is China. If China can't stop the current outbreak like other countries have, China will continue to seed the rest of the world periodically with this disease."

Most immediately, officials are alarmed by reports that the number of cases of severe acute respiratory syndrome has more than doubled in the past week in neighboring Taiwan, which could become the next major crisis point. Yesterday, Taiwan reported its first SARS death.

The most intense concern is focused on the less-developed parts of Asia and Africa, which have far less effective health systems to identify and isolate cases, and vast populations especially vulnerable to new infections because of AIDS.

But new outbreaks could occur at anytime anywhere in the world where an infected person carries the virus, including the United States, officials said.

"You are just one or two people away from a worldwide epidemic," said Georges Benjamin, executive director of the American Public Health Association. "It's like you've had a big fire, and put the big fire out. The fire department needs to stick around and make sure the fire is really out."

Because SARS seems to have been contained in Hanoi, and may be under control in Toronto, Singapore and Hong Kong, health authorities are becoming increasingly concerned that the world may become complacent about the disease, which has struck 5,663 people in 26 nations and killed 372. At a briefing yesterday, U.S. health officials said the SARS threat has not abated and they were working urgently to prepare for the next onslaught.

"If a large city around the country becomes the next Toronto . . . we need to be able to support that city or multiple cities with equipment and personnel," said Jerome Hauer, acting assistant secretary for public health preparedness at the Health and Human Services Department. "We are looking at this as a real-time issue right now and how we would respond tomorrow, next week or next month."

SARS has overwhelmed hospitals everywhere there has been an outbreak, including Beijing, where the mayor yesterday warned that panic is setting in.

As part of the U.S. preparation, the federal government is buying 3,000 more respirators for its stockpile of emergency medical supplies that can be quickly distributed to cities overwhelmed with patients, officials said. There are about 100,000 ventilators in the United States, but at least 80 percent are in use at any time.

"We're not out of the woods yet," HHS Secretary Tommy G. Thompson said.

At least 52 probable cases of SARS have been reported in the United States, including three in Virginia, and health officials are monitoring 222 suspicious cases.

Disease experts also noted that epidemics often occur in waves, with successive incarnations becoming more intense. The devastating 1918-19 Spanish flu, for example, started in the spring and seemed to subside, only to reemerge in a more deadly form at the end of the summer.

"If you look at the epidemic curve for that 1918 epidemic, we had an early, very modest blip and then pretty much a rapid and significant decline during the summer months," said Alfred Sommer, dean of the Bloomberg School of Public Health at Johns Hopkins University. "People started patting themselves on the back. Then an explosive [reemergence] come wintertime."

In addition, many viruses, but especially respiratory viruses, follow a seasonal pattern, subsiding in warm weather and reemerging when it turns cold. SARS first emerged in southern China in November and could be nearing the end of a seasonal outbreak. The disease is caused by a previously unknown coronavirus; other coronaviruses cause the common cold, which tends to follow a seasonal pattern.

"There are two aspects of a seasonal component -- one bad and one good," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "The good one is that it goes away naturally. The bad is it comes back."

Added Benjamin: "We should not be surprised if it comes back with a vengeance."

As a result, the worldwide network of laboratories that tracks flu viruses to determine the best vaccine formula and to spot the emergence of any particularly dangerous strains will begin doing the same thing for the SARS virus, officials said.

"As we survey for the emerging new strain or drifted strain or shifted strain of influenza, the same thing will be applied in anticipation of the possibility that this may be a seasonal thing," Fauci said.

Health experts worry that China's health system is ill-equipped to fight the epidemic, and the scenario they fear most is that SARS will rage out of control there and then spread to someplace like Africa.

"To date, this disease has primarily been limited to larger cities in the developed world. It's a whole different challenge if this gets into the densely populated areas of the developing world," said Michael T. Osterholm of the University of Minnesota. "Many people in the developing world live in conditions that rival what Dickens wrote about hundreds of years ago."

The ongoing concern comes as the death rate for SARS appears to increase. In Geneva, Mark Salter of the WHO said the death rate had risen to between 6 percent and 10 percent.

Surprisingly, the highest death rates appear to be occurring in the most advanced parts of the world.

Salter speculated that the higher death rate in places such as Toronto and Hong Kong may be because more victims in those places were health care workers, who may have gotten a larger dose of virus because of their close contact with patients.

Salter also said he would investigate reports from Hong Kong that some patients who appeared to recover later relapsed.

There is no proven treatment for SARS, but some doctors are using the antiviral drug ribavirin combined with steroids.

Salter speculated that the relapses could be because patients were taken off steroids too soon. Evidence is mounting that much of the illness is caused by damage from the immune system's attempt to fight off the infection.

Canada's federal health agency issued a warning to physicians to be aware of side effects of ribavirin, saying, "Physicians currently treating patients with ribavirin are strongly advised to critically reexamine the risk/benefit for each patient before continuing treatment." Serious reactions to the drug have been reported.

Canada yesterday announced the deaths of two men, ages 39 and 72, as the result of earlier SARS infections.

Health officials have said several SARS patients in Toronto were in critical condition, making further deaths likely.

Staff writers Justin Gillis in Washington and DeNeen L. Brown in Toronto contributed to this report.