KIKWIT, Zaire -- AFTER 2 1/2 GRUELING weeks, David Heymann let a momentary ear-to-ear grin transform his usually stern face, grabbed a glass of ice cold Primus beer and, after a long, noisy gulp, let out a sigh of deep satisfaction. "We did it. We beat the virus," Heymann chuckled, pulling back another long draught of the first genuinely cold liquid he had had since he arrived in Zaire 16 days earlier to lead the World Health Organization's Ebola control effort.

On that day, May 10, people who saw Heymann say his face was a strained, emotionless mask, a mask he now admits hid a growing feeling of horror as he surveyed the hospital where an outbreak of Ebola would grow and be nurtured under medical conditions that most countries would consider intolerable.

Heymann arrived at the hospital accompanied by a WHO colleague, Mark Szczeniowski, and Dr. Tamfun Muyembe -- three men who were about as qualified for this job as any three people could possibly be.

Heymann, an American epidemiologist trained by the U.S. Centers for Disease Control, and Muyembe, a leading virologist at the University of Kinshasa, both had been members of a medical team that fought a 1976 Ebola outbreak that had left more than 300 dead and given the world its first knowledge of the incurable disease. Szczeniowski, a technical officer, had spent 19 years in Zaire in the '60s and '70s battling smallpox.

All three knew, going in, that Zaire presented tremendous infrastructural obstacles, but the two Americans would soon learn that the Zaire they remembered, the Zaire of the 1970s, had taken a turn for the worse.

The scene that greeted the men at Kikwit General Hospital, the epicenter of the breakout, was more horrific than anything the WHO scientists had imagined during their flight from Geneva.

"There was blood everywhere," Heymann recalled last week."Blood on the mattresses, the floors, the walls. Vomit, diarrhea . . . wards were full of Ebola cases. [Non-Ebola] patients and their families were milling around, wandering in and out. There was lots of exposure."

And lots of death. From mid-January to the end of May, the Ebola virus had infected at least 160 people in Zaire, killing 121. Passed through bodily fluids, the virus became an overachiever in a country in which many of the most basic services -- running water, sewage control, electricity -- had been damaged or lost during running outbreaks of civil strife.

"The women mourners sat right here," Heymann recalled, pointing to a slab of concrete walkway that led from the wards full of Ebola patients to the morgue. "And you'd see family after family rocking and wailing, facing that building."

When Heymann and Szczeniowski walked into the hospital on May 10, it had no running water, only sporadic electricity and virtually no supplies. There were no bed linens; few of the coiled springs and bed frames had mattresses. Most patients, in fact, preferred to lie on the floor, their blood leaving obvious stains on the concrete.

The staff, all civil servants, had gone unpaid for four years and all of them worked odd jobs on the side, sold pilfered hospital supplies or had other hustles that provided them with funds for food and housing.

"There were people dying everywhere," he added, "and the women were wailing. It was surreal. They were filling all the graves -- these Red Cross volunteers who had absolutely no protection. And there was this amazing good will, with the doctors and nurses working without any protective gear whatsoever. Incredible good will.

"And we realized," he said, "that this was not like Yambuku, where the epidemic was pretty much over by the time we got there. We were right in the middle of it."

Heymann and Szczeniowski felt excited and, Heymann confessed, "terribly inadequate in the face of the problem."

Known among his colleagues as a cool-headed and focused thinker, Heymann speaks fluent French and possesses the ability to juggle numerous reponsibilities and personnel at once. Szczeniowski is, in WHO parlance, a technical officer whose job is logistics.

That first day, the two WHO workers sat with Muyembe, pooling their considerable experience to plot a strategy for what Heymann would call task number one: Stop the epidemic.

The Zairean Muyembe would become the touchpoint for all of the international efforts in Kikwit -- an effort that would bring together a community of infectious disease experts from around the world, many of whom knew each other, had faced similar situations in the past, and comfortably moved into pre-ordained roles as if they had worked there all their lives.

In fact, many had worked in places much like this for much of their professional lives.

A good-humored, occasionally bombastic man, Muyembe is fluent in English, French and three Zairean languages -- and is polite and kind in all of them. It was, many of the scientists agreed, a personality strength that would make him the perfect leader for what was to come.

Muyembe wanted a passive surveillance operation set up in the hospital and local clinics to screen Ebola cases. He wanted all non-Ebola treatment at the general hospital shut down and a system of sanitary isolated care established for Ebola victims.

In Kikwit and the nearby villages, he called for a much more active surveillance program to be put in place to seek out contageous cases. And he wanted to find out how the virus was transmitted, and to immediately begin setting up improved sanitation and public education to stop the virus.

The leadership role was a natural one for Muyembe for a second reason, as well. He had been the one, in fact, that first raised the cry that eventually resulted in international teams of doctors and scientists responding.

In mid-April, Muyembe had left Kinshasa to investigate an alleged epidemic of bloody diarrhea in Kikwit, a city of between 300,000 and 500,000 people some 300 miles from the capital. When he reached Kikwit hospital, he was appalled by the death toll and despair the diarrhea epidemic had wrought.

Physicians in Kikwit General Hospital were in a state of panic. Not only were their patients dying despite antibiotic therapy, but the medical staff and nuns were falling victim to the mysterious ailment. A tentative diagnosis of Shigella -- a bacterial disease that was normally 30 percent fatal, but should have been curable with antibiotics -- was assigned to the crisis.

But Muyembe wasn't so sure.

He radioed an urgent message to Sister Agnes, an octogenerian former Kikwit nun and pharmacist living in retirement in Brussels, asking for help: It may be a Shigella strain, he wrote, that is resistant to first-line antibiotics. He asked her to send ciprofloxacin, one of the most powerful and expensive antibiotics on the market.

He also mentioned that the cases reminded him of an epidemic he had seen in 1976 in Yambuku, the country's first Ebola breakout.

Unable to quickly raise the cash for enough ciprofloxacin to handle such an outbreak, Sister Agnes contacted Dr. Simon van Nieuwenhove, of Antwerp's Institute of Tropical Medicine, reading him Muyembe's plea.

The word Ebola stood out for van Nieuwenhove. He, too, had been involved in the 1976 Yambuku outbreak, his task being to drive all over central Zaire in search of other cases of the then-mysterious disease. It seemed incredible to van Nieuwenhove that the virus would re-emerge hundreds of miles from Yambuku after a 19-year hiatus. But the possibility was there.

His response to Sister Agnes was blunt: Tell Muyembe to send blood and tissue samples. Immediately.

The samples arrived in Antwerp on May 6, but were quickly sent to the American Centers for Disease Control and Prevention in Atlanta. If it was Ebola, van Niewenhove and officials from the World Health Organization agreed, then it should be handled in the most secure facility available.

By May 9, Dr. C.J. Peters, chief of the CDC's Special Pathogens branch, was on the phone to Dr. James LeDuc in Geneva, head of the WHO's infectious disease branch, telling him that their worse nightmares were coming to fruition -- Ebola was on the loose again in Zaire.

Heymann and Szczeniowski were immediately dispatched. With the broad outlines of the battle sketched in, other volunteers began to arrive soon afterward -- including Dr. Barbara Kiersteins and her team of two volunteers from Medecins sans Frontieres (Doctors Without Borders).

MSF is a private, nonprofit organization of voluntary physicians and a small corps of paid staff with headquarters in Amsterdam and Brussels. Because it is not affiliated with any government or UN organization, MSF has been able to operate in politically precarious situations, and has alerted the world to numerous epidemics and famines that had been kept secret by the respective governments.

Kiersteins^ responsibility was to bring order to the chaos at Kikwit Hospital. A veteran commander of such medical horrors as Rwanda's refugee camps, Kiersteins was accustomed to making quick, deliberate decisions.

"The hospital was in a sorry state," she told a reporter who had arrived just four days after she had. "The patients were in a sorrier state. The staff had no protection and they hadn't been paid for risking their lives. So we decided to focus on hospital sanitation and establishment of an isolation ward."

On Thursday, May 11, Kierstein's crew began hooking up the hospital's ancient water system, but gave up after realizing that all the pipes were blocked and rusted. Instead, they set up a plastic rainwater collection and filtration system. A plastic cordons sanitaire -- a thin, plastic wall, really -- was set up, isolating a ward for Ebola patients. And they dispersed gloves and masks to the hospital staff.

By Friday night, Kiersteins decided to seek additional help. Her team spent Saturday morning listing essential supplies, using a satellite telephone to pass the list onto Brussels: "Send respirator masks, latex gloves, protective gowns, disinfectant, hospital linens and plastic mattress covers, plastic aprons, basic cleaning supplies and cleansers, water pumps and filters, galoshes, tents . . . "

"I have seen many African countries, and even compared to others, this was shocking," Kiersteins said. "So when you see how they [the hospital staff] coped -- well, the only thing they had to work with was their brains."

For 26 days, however, the brains, guile and dedication of the on-site rescue teams -- as well as the numerous Zairean volunteers and medical workers -- continued to be their main weapon. The supplies did not begin arriving in suitable supply until May 27.

One thing Kiersteins did have, however, was cash. To provide incentive to keep them on the job -- despite the obvious and considerable risks -- Kiersteins team began paying the hospital staff.

While Kiersteins^ crew was working around the clock at the hospital, Heymann, Szczeniowski and Muyembe gathered college and medical students to train in the techniques of active surveillance. Teams of students were organized and, by Friday, were combing Kikwit for cases with Ebola symptoms.

One member of that team, an accounting student named Jean Bosco Katshunga, has since been credited with discovering what may be the first chain of transmission for the disease when he noticed a series of funerals being held by one of his neighbors. He interviewed the neighbors and passed the information along to the international team.

They have since made investigation of this chain one of the key elements of their overall probe.

From this point on, the international team would quickly swell in size, with more than 30 experts pouring in from all over Europe, the United States and Africa. Each new arrival would be assigned to an appropriate team and most were absorbed into the effort with remarkable ease.

On Friday, for instance, Pierre Rollin of the American CDC and Dr. Phillipe Calain of the French Institut Pasteur arrived in Kikwit. Calain immediately took over all Ebola patient care in Pavilion No. 3, the isolation ward, while Rollin became part of the active surveillance team.

In 1989-90, Rollin worked with the U.S. Army's pathogen group at Fort Detrick, where he helped investigate the outbreak of Ebola among monkeys at a commercial primate center in Reston, Va.

Belgian physician Robert Colebenders of the Institute of Tropical Medicine in Antwerp arrived soon afterward and took over the hospital's emergency room. The CDC's Dr. Ali Khan and WHO's Dr. Guenael Rodier commanded the active surveillance effort. Dr. Robert Swanepoel of the National Virology Institute in Johannesburg and veteranarian Oyewale Timori of WHO's office in Harare, Zimbabwe, began the search for the animal source of the virus.

Khan, a deadly serious, Brooklyn-born epidemiologist, slogged through the tedious detective work of trying to identify who passed the virus to whom. Despite the sweltering heat, Khan always wore an ironed shirt and tie.

"I always wear a tie, no matter where I am," Khan explained. "I want to show respect for the people I interview." The only non-French speaker on the team, Khan had a medical student by his side at all times, translating his orders and explaining what people were saying.

On May 20, Heymann satellite-phoned Geneva to say he was concerned a new wave of cases -- a fourth wave -- could emerge if the medical community did not keep the pressure on. In response, a team of Swedish doctors quickly arrived to support those already working in the field and replace those who needed to leave.

Finally, on May 27 -- just hours after Heymann left Kikwit, destined for Geneva -- a Hercules military transport jet, loaded with the requested supplies from Sweden, landed at Kikwit airport.

With the pace of the disease nearing a standstill, Heymann finally kicked back and drank his beer. The effort would go on without him, with new teams of CDC and WHO investigators continuing to track the epidemic backwards in time, trying to find the exact moment when the virus jumped from its original host -- perhaps an animal deep in the jungle -- into man.

Yesterday in Geneva, Heymann greeted the European press corps, reiterating the good news that the epidemic was over.

The numbers of cases had reached 205, with 153 deaths, and would appear to keep rising over coming days, Heymann said. But the higher numbers wouldn't reflect new active cases of the disease in and around Kikwit: rather, they would represent historic cases, dating back to January, that scientists are now finding through painstaking examination of hospital records and family interviews.

But even as Heymann prepared the long-term investigation strategy that will hopefully reveal the entire history of Kikwit's epidemic, officials in Zaire said that bloody diarrhea has broken out elsewhere in Kikwit's province of Bandundu.

In an area 470 miles north of Kikwit, a town called Tendjua, 25 people have died from the ailment, according to Zaire's Health Ministry spokesman Dr. Bompenda Bonkumo.

As was the case in Kikwit, experts are tentatively assigning a diagnosis of Shigella to the illnesses, and WHO did not express concern that they might be due to Ebola.

According to wire service accounts yesterday out of Kinshasa, WHO's representative in the country, Dr. Abdou Moudi, said that there was an "alarming" number of epidemics in Zaire, and his office was having trouble getting accurate information on any of them.

"There are epidemics everywhere," Moudi told Reuters. "The system of reporting epidemics is not good. You have trained people but they don't have the means [to communicate]. If you don't have the information, you will intervene only when the fire has caught and that is too late."

The scientists and doctors have beaten the threat in Kikwit, they know, but other threats remain hidden in the secret jungle, diseases that could -- at any point -- re-emerge in ways that man could not yet understand.

Jamie Talan contributed to this story