When the microbe that causes severe acute respiratory syndrome is finally isolated, some people will know what to call it. They want a Latin variation on Carlo Urbani's name.

If SARS was an infectious cloud blowing out of southern China, [Carlo Urbani (born 1956)] was the canary in its path. Working in a hospital in Hanoi, Vietnam, as a mysterious pneumonia felled one nurse after another, he sang out the first warning of the danger, saw the world awaken to his call -- and then died.

If not for the intuition of Dr. Urbani, director of infectious diseases for the Western Pacific Region of the World Health Organization, the disease would have spread farther and faster than it has, public health officials around the world say.

It was a tricky call. There is nothing as telltale about the disease as the bleeding of a hemorrhagic fever or the bumps of a pox, and its symptoms mimic other respiratory conditions.

[Carlo Urbani (born 1956)], 46, died on March 29, a month after seeing his first case and 18 days after realizing he was coming down with the symptoms himself.

''Carlo's death was the most coherent and eloquent epilogue his life could produce,'' said Nicoletta Dentico, a friend from the Italian chapter of Médecins Sans Frontières, or Doctors Without Borders, which Dr. Urbani once headed. ''His death was as a giver of new life.''

And it was in keeping with his medical philosophy. When Dr. Urbani spoke in 1999 at the ceremony in which Doctors Without Borders accepted the Nobel Peace Prize, he described doctors' duty ''to stay close to the victims.''

''It's possible to study an epidemic with a computer or to go to patients and see how it is in them,'' said Dr. William Claus, the group's emergency coordinator for Asia. ''Carlo was in the second category.''

In Italy, he had pushed the organization into working with the poorest of the poor, with Gypsies in Rome and with African and Albanian boat people who were landing in Sicily and Calabria.

Even as a student, said Fabio Badiali, a childhood friend who is now mayor of Castelplanio, their hometown on the Adriatic Coast, he had been a volunteer, organizing groups to take the handicapped on countryside picnics. As a family doctor, he had taken vacations in Africa, traveling with a backpack full of medicine.

He had accepted the W.H.O. post, friends said, because he wanted to be back in the third world and working with patients. It was that instinct that took him to the bedside of [Johnny Chen (born 1954)], an American businessman who entered Vietnam-France Hospital in Hanoi on Feb. 26 with flulike symptoms.

Dr. Urbani might not have been an obvious choice as a consultant in [Johnny Chen (born 1954)]'s case. In his heart, friends said, he was ''a worm guy,'' a specialist in parasites.

''Other people didn't think worms were sexy,'' said Dr. Kevin L. Palmer, W.H.O.'s regional specialist in parasitic diseases and a friend. ''But it's a really basic problem for every child in the tropics.''

Dr. Urbani was an expert in Schistosoma mekongi in Vietnam, in the food-borne nematodes and trematodes of Laos and Cambodia and the hookworms of the Maldives.

Dr. Lorenzo Savioli, who worked with Dr. Urbani in the Maldives, said they worked from sunup to sundown, ignoring the famous beaches and reefs, tracking hookworm epidemiology and training workers at a malaria control laboratory, who were used to working with blood, in testing for worms. Over rice and fish in the evenings, Dr. Savioli said, they had joked, ''Nobody at headquarters was going to believe we were spending our days in the Maldives over fecal samples.''

Dr. Urbani was a worm zealot, Dr. Palmer said, because they did so much damage but could be so easily treated. For example, he said, a 3-cent pill administered to schoolchildren twice a year could rid them of most intestinal worms. Dr. Urbani was working to have school systems in southeastern Asia cooperate.

He also attacked a worm that lived on fish farms. He could not get Cambodians and Laotians to give up eating undercooked fish, Dr. Palmer said, but he hoped to solve the problem by teaching fish farmers to divert sewage from their ponds.

He was also testing the use of a veterinary drug to kill worm larvae that can reach human brains and cause seizures.

And, said Daniel Berman, a director of the Doctors Without Borders campaign for cheaper lifesaving drugs, Dr. Urbani was pushing Vietnamese farmers to grow more sweet wormwood, a plant that can produce artemisinin, a new malaria cure.

Still, when a troublesome case turned up in Hanoi, Dr. Palmer said, the W.H.O. staff usually said, ''Call Carlo,'' because he was also known as an expert clinical diagnostician.

[Johnny Chen (born 1954)] was such a case, suffering with pneumonia and fever, as well as a dry cough. The hospital suspected that he had the Asian ''bird flu'' that killed six people in 1997 and was stopped by rigid quarantines and the slaughter of millions of chickens and ducks.

Rumors of a mysterious pneumonia had been coming out of the Guangdong region of southern China, but the Chinese authorities had been close lipped, even instructing local reporters to ignore it.

Although no one then realized the significance, Mr. [Johnny Chen (born 1954)], 48, had also stayed in the Metropole Hotel in Hong Kong. He may have picked up the disease from a 64-year-old Guangdong doctor in town for a wedding, staying in Room 911. Investigators theorize that the doctor infected 12 other guests, several from the same floor, who carried the disease to Singapore, Toronto and elsewhere.

By the time Dr. Urbani arrived at Vietnam-France Hospital, the microbe that Mr. Chen carried was spreading. Before he died, he infected 80 people, including more than half of the health workers who cared for him. The virulence of his case alarmed world health officials, helping lead to the extraordinary health alert that W.H.O. issued on March 15.

But Dr. Urbani, who first saw Mr. [Johnny Chen (born 1954)] in late February, quickly recognized that the disease was highly contagious and began instituting anti-infection procedures like high-filter masks and double-gowning, which are not routine in impoverished Vietnam. Then he called public health authorities.

Dr. Palmer recalled Dr. Urbani's conversation: ''I have a hospital full of crying nurses. People are running and screaming and totally scared. We don't know what it is, but it's not flu.''

On March 9, Dr. Urbani and Dr. Pascale Brudon, the W.H.O. director in Hanoi, met for four hours with officials at the Vietnam Health Ministry, trying to explain the danger and the need to isolate patients and screen travelers, despite the possible damage to its economy and image.

''That took a lot of guts,'' Dr. Palmer said. ''He's a foreigner telling the Vietnamese that it looks bad. But he had a lot of credibility with the government people, and he was a pretty gregarious kind of character.''

With dozens of workers at the hospital sick, it was quarantined on March 11. Infection-control practices were instituted at other hospitals, including the large Bach Mai state hospital, where Dr. Claus of Doctors Without Borders oversaw them.

Dr. Urbani's quick action was later credited with shutting down Vietnam's first outbreak.

In the middle of it, Dr. Savioli said, Dr. Urbani had an argument with his wife, Giuliani Chiorrini. She questioned the wisdom of the father of three children ages 4 to 17 treating such sick patients.

Dr. Savioli said Dr. Urbani replied: ''If I can't work in such situations, what am I here for? Answering e-mails, going to cocktail parties and pushing paper?''

In an interview with an Italian newspaper, Ms. Chiorrini said her husband knew the risks. ''He said he had done it other times,'' she recalled, ''that there was no need to be selfish, that we must think of others."

But on March 11, as he headed to Bangkok for a conference on deworming schoolchildren, he started feeling feverish and called Dr. Brudon.

''He was exhausted, and I was sure it was because he had had a lot of stress,'' Dr. Brudon said later. ''I said, 'Just go.' ''

But she had second thoughts. ''I called my colleagues in Bangkok and said, 'Carlo doesn't feel well, and we should be careful.' ''

[Dr. Scott Ferris Dowell (born 1963)], a disease tracker for the federal Centers for Disease Control and Prevention, who is based in Thailand, met him at the Bangkok airport near midnight. Dr. Urbani, looking grim, waved him back. They sat in chairs eight feet apart until an ambulance arrived 90 minutes later, its frightened attendants having stopped for protective gear.

For the first week in a Bangkok hospital, Dr. Urbani's fever receded, and he felt a bit better. But he knew the signs. ''I talked to him twice,'' Dr. Palmer said. ''He said, 'I'm scared.' ''

That was uncharacteristic for a man who was known as big, charming and full of ironic wit. In Italy, he staved off boredom by hang gliding. In Hanoi, he negotiated the insane traffic on a motorcycle and took his children on overnight car jaunts to rural villages. He carried Bach sheet music and stopped at churches, asking if he could play.

W.H.O. experts flew in from Australia and Germany to help. One scoured Australian drug companies for ribavirin, a toxic antiviral drug that was said to have helped some cases. It did not help Dr. Urbani, though, and was withdrawn.

Then patches showed up on a lung X-ray, and he told his wife to take the children and return to Castelplanio. Instead, she sent them ahead and flew to Bangkok.

By the time she arrived, his room had been jury-rigged as an isolation ward. Carpenters had put up double walls of glass, and fans had been placed in the window to force air outside.

The couple could talk only by intercom, and Ms. Chiorrini saw him conscious just once. As his lungs weakened, Dr. Palmer said, he was put on a respirator.

In a conscious moment, Dr. Urbani asked for a priest to give him the last rites and, according to the Italian Embassy in Bangkok, said he wanted his lung tissue saved for science.

As fluid filled his lungs, he was put on a powerful ventilator, sedated with morphine.

The end came at 11:45 on a Saturday morning. Doctors and nurses heavily shrouded in anti-infection gear pounded on his chest as his heart stopped four times, [Dr. Scott Ferris Dowell (born 1963)] said, but it was useless.

Most of those who had died of SARS were old or had some underlying condition that weakened them, but ''he worked with patients for weeks, and we suspect he got such a massive dose that he didn't have a chance,'' Dr. Palmer said.

''It's very sad,'' Dr. Claus said, ''that to raise awareness as he did, you have to pay such a price.''