Carlo Urbani, the scientist who discovered the first clue that a dangerous new microbe was beginning to spread around the globe, succumbed yesterday in a Bangkok hospital bed to the frightening disease he alerted the world to.

It was Urbani, an Italian epidemiologist at the World Health Organization's office in Hanoi, who first responded last month when anxious hospital officials phoned to report that a sick U.S. businessman was infecting doctors and nurses with a strange pneumonia. Within days, Urbani himself fell ill.

"Carlo was the one who very quickly saw that this was something very strange. When people became very concerned in the hospital, he was there every day," said Pascale Brudon, the WHO representative in Hanoi. "We are all devastated."

The pneumonia, now known as severe acute respiratory syndrome, or SARS, has become the focus of a worldwide health emergency. Yesterday, U.S. health officials warned against any unnecessary travel to Hanoi, Hong Kong, Singapore and all of China, and outlined stringent new measures to keep infected people from spreading the sometimes fatal disease.

"We are concerned about the possibility of airborne transmission," said Julie Gerberding, director of the Centers for Disease Control and Prevention. "We are very concerned about the spread of this virus. We may be in the very early stages of what could be a much larger problem."

The disease is the first dangerous new infection that spreads directly from one person to another to emerge in decades. The story of how it surfaced, and how Urbani and the rest of the world responded, has profound implications for humanity's readiness to fight dangerous new germs, whether they evolve naturally or are grown deliberately in a bioterrorist's lab.

"If there's an influenza pandemic, or if there's a deliberate release of a biological weapon, there will be many valuable lessons from this experience," said David L. Heymann, head of WHO's communicable diseases program. "This is really the first time we've attempted something on this scale, and the global community has pulled together."

By the time the sick Chinese American businessman, Johnny Cheng, made it to the French Hospital in Hanoi on Feb. 26, he was already deathly ill. It looked like pneumonia, but Cheng's high fever, cough and other symptoms worried doctors. Cheng had just flown in from Hong Kong, where a father and son had recently died from the "bird flu," a deadly bug that experts long feared might spark a pandemic rivaling the Spanish flu of 1918-19, which killed as many as 50 million people.

Urbani called his regional supervisor in Manila.

"He was reporting a probable case of bird flu," said WHO spokesman Dick Thompson, who happened to be where the phone rang on March 5. He was sitting at the desk of a colleague who had been dispatched to Beijing to persuade officials to be less secretive about a mysterious outbreak in Guangdong province in southern China. Since early February, WHO had been hearing reports that hundreds of people were getting sick with what sounded like a strange form of pneumonia. Officials worried that it, too, might be bird flu.

Unbeknownst to health authorities at the time, Cheng, 48, had traveled in Guangdong and is believed to have stayed at the Metropole Hotel in Hong Kong during the same time as a 64-year-old Chinese professor who had been treating patients in Guangdong. The professor, in Hong Kong for a wedding, had checked into Room 911.

Investigators later determined that he infected 12 other guests in the hotel, perhaps by coughing while waiting for an elevator. Among them were several tourists, who then spread the disease to Singapore, Hong Kong, Toronto and elsewhere. The professor died in a Hong Kong hospital after infecting two family members and four hospital workers.

But in early March, neither Urbani nor anyone else knew any of that. So over the next week, "Carlo kept going back to the hospital" in Hanoi to take samples, Thompson said. "Carlo kept going back and working with the staff there, who themselves started to get sick. And then Carlo got sick."

As the number of sick hospital workers in Hanoi mounted, WHO alerted its Global Outbreak and Response Network, an international collaboration set up at its Geneva headquarters three years ago specifically to spot the first signs of what could be the next pandemic. As part of the network, a Canadian computer program scours everything from newspaper reports to Internet chat rooms every day, searching for telltale clues.

"Every morning we have a meeting in a large conference room. We discuss whatever reports of outbreaks have come in: 'How they are being tracked? What's being done about them?' " Thompson said.

With the number of sick hospital workers in Hanoi rising, Cheng, the businessman, was transferred back to a hospital in Hong Kong. On March 13 he died, but not before infecting more hospital workers. WHO sent an official from Beijing to Hanoi to investigate.

By March 11, dozens of health care workers at the Hanoi hospital were sick, including several who were critically ill, and the facility was overwhelmed. Officials shut it down and imposed a quarantine. Urbani had been transferred to a hospital in Bangkok

WHO assembled four international teams to help medical workers cope, track the spread of the disease, search for its cause and find a way to fight it.

With another outbreak now affecting dozens of hospital workers in Hanoi and Hong Kong, WHO issued a very unusual alert on March 12, warning that a mysterious flulike illness appeared to be spreading in parts of Asia. The cause was unknown, but it appeared to be highly contagious. But with war looming in Iraq, the alert received little attention.

Over the next few days, the number of cases in Hanoi continued to mount, and WHO started getting reports of outbreaks at more hospitals in Hong Kong, as well as in Singapore. Three Singapore women who had stayed at the Metropole fell ill after they returned home, apparently infecting dozens of doctors and nurses, who in turn infected dozens more. Other possible cases were being reported in Thailand and possibly the Philippines.

In Hong Kong, people were starting to panic, rushing to buy protective masks.

The crisis burst into public attention on March 15, when WHO received word of the first possible cases outside Asia. An elderly Toronto woman who had stayed at the Metropole had gotten sick and died after returning home. She had infected several family members and their doctor. A son had already died. A daughter, who also was infected, had just gotten back from Atlanta. That raised fears, which never materialized, that the disease had been spread there.

The night before, a 32-year-old doctor who had been treating sick patients in Singapore had called a friend from New York to say he was feeling ill. He had been attending a medical conference and decided to fly home. When WHO was notified, officials alerted authorities in Germany, who intercepted the plane during a stopover in Frankfurt, removed the sick doctor, his wife and mother, and put them in isolation.

In New York, health officials began looking for any signs the disease had spread there.

WHO declared that the disease now posed a "worldwide health threat."

Over the next few days, more than 200 reports of possible cases streamed in from Asia, Europe, Canada and, for the first time, the United States.

The CDC joined a network of 11 laboratories around the world that WHO organized in a crash effort to identify the cause. Every test for known pathogens came back negative, raising the specter that a previously unknown microbe was spreading. Antibiotics appeared useless.

Health authorities in Asia started screening air travelers for symptoms before they let them board planes. In the United States, anyone getting off a plane or boat from Asia who seemed sick was immediately isolated. Every other passenger was handed a card with instructions to watch for symptoms.

WHO officials suspected the disease had originated in Guangdong, but Chinese officials had acknowledged only 305 cases and five deaths and indicated that the epidemic there was over. Worried that the outbreak was much larger and ongoing, WHO quietly pressed for details

The good news was that the disease did not appear to be spread casually. Everyone who had gotten sick, whether medical workers or family workers, had had close contact with an infected person. But there were chilling reports of individuals who appeared to be highly infectious, apparently spreading it on planes and possibly even in apartment complexes. Singapore Airlines announced Friday that a flight attendant on the plane that was stopped in Frankfurt because of the sick doctor had SARS.

WHO organized a network of doctors treating patients around the world so they could exchange information. Some doctors reported that the antiviral drug ribavirin seemed to help some patients.

Scientists zeroed in on a previously unknown virus as the probable cause of the outbreak. Evidence seemed to be mounting that the culprit was a coronavirus, which usually causes nothing more serious than the common cold. If the cause was a coronavirus, it must have either mutated into something far more deadly or jumped from an animal. Scientists began developing tests to detect the virus.

After weeks of pressure, China finally acknowledged that the outbreak in Guangdong had affected many more people -- at least 792, including 31 deaths -- and was not over. In fact, it had spread, to Beijing among other places. Chinese doctors pledged to work more closely with international health officials.

By yesterday, the worldwide toll stood at 1,550 cases in 13 nations, including 62 in the United States, and 54 deaths. Hong Kong was especially hard hit, with at least 470 cases and 10 deaths.

Hong Kong and Singapore shut their schools after cases began appearing among children, and quarantined more than 2,500 people suspected of carrying the disease. Canadian officials declared an emergency in Ontario and quarantined a hospital that appeared to be a hot spot for the outbreak.

WHO officials expressed hope that the international containment efforts had stemmed the epidemic outside Asia, but whether the epidemic was waning or just beginning remained far from clear.

"It's very interesting how the world rallied around this," WHO's Heymann said. "The disease has spread to other countries, but we're hopeful that we've been able to stop major transmission around the world.

"The question is now whether this will become another human pathogen that will now remain with us. But this is certainly good practice if there is a major influenza outbreak, or if there's a deliberate release of a biological weapon."

Some patients in Hanoi had started to get better, indicating that the disease was not uniformly fatal, and no new cases were reported there in days.

Urbani, 46, who was married with three children, was an expert in parasitic diseases of schoolchildren and had worked in Cambodia, Laos and Vietnam.

"Carlo Urbani's death saddens us all deeply at WHO," said Gro Harlem Brundtland, WHO's Director-General. "His life reminds us again of our true work in public health. Today we should all pause for a moment and remember the life of this outstanding physician."

In Hong Kong, several hundred cases of severe acute respiratory syndrome, including about a dozen deaths, have prompted people to wear masks.Epidemiologist Carlo Urbani died in the epidemic he first detected.