On the flight back to Atlanta, Dr. Pierre Rollin snoozed in Seat 26C in his usual imperturbable way, arms folded, head bobbing, oblivious to loudspeaker announcements and the periodic passing of the galley cart.

This routine had become part of his lore. During each viral outbreak, Rollin, the top Ebola expert at the Centers for Disease Control and Prevention, would outlast his younger colleagues in the hotel lobby, staying awake until 3 or 4 a.m. to plug new cases into a database. He managed to do this at 61 because he possessed an uncanny ability to sleep anywhere anytime, whether on the hardwood floor of a staff house in Zaire (Ebola, 1995) or in a back seat lurching down a cratered road in Madagascar (Rift Valley fever, 2008).

On this trip home from Guinea on May 7, Rollin found himself at particular peace. His 5 1/2-week stay as the CDC’s team leader in the opening days of Guinea’s effort to control Ebola had gone about as well as one could have hoped.

The number of Ebola cases reported each week had been declining steadily for a month. It had been more than 10 days since doctors had seen a new patient in Conakry, the capital, where Rollin worked alongside other early responders from the World Health Organization and Doctors Without Borders.

New patients had slowed to a trickle in the Forest Region of southeastern Guinea, the center of the outbreak, and there had not been a report across the border in Liberia for four weeks. Sierra Leone, although surrounded by Guinea and Liberia, had not discovered a single confirmed case.

Like the 10 Ebola crises he had handled before, in Uganda and Sudan and the Democratic Republic of Congo, this first substantial outbreak in West Africa seemed to be burning itself out after a few months and a few hundred infections.

"This is close to over," Rollin told himself, a view common among the virus hunters. "That’s it for this outbreak."

Or so he thought. In fact, Rollin and other well-intentioned veterans of past Ebola campaigns had tragically underestimated this outbreak, overlooking clues that now seem apparent. Viewing the West Africa epidemic through the prism of nearly two dozen previous outbreaks across the continent, they failed to appreciate that the 2014 version would be unique in catastrophic ways.

After more than 20,000 cases and 7,800 deaths, it can be hard to recall that there was a moment in the spring when the longest and deadliest Ebola outbreak in history might have been stopped. But without a robust and coordinated response, an invisible epidemic was allowed to thrive alongside the one assumed to be contained.

Although conditions were ideal for the virus to go underground, some of the world’s most experienced Ebola fighters convinced themselves that the sharp decline in newly reported cases in April and May was real.

Tracing those exposed to Ebola and checking them for symptoms, the key to containing any outbreak, had been lacking in many areas. Health workers had been chased out of fearful neighborhoods. Ebola treatment centers had gained such reputations as deathtraps that even desperately ill patients devoted their waning strength to avoiding them.

With the affected countries often lacking the most basic medical infrastructure, the health care challenges proved staggering. But the most tragically missed opportunities stemmed from the poor flow of information about who was infected and whom they might have exposed.

A two-month investigation by The New York Times into this largely unexamined period discovered that the WHO and the Guinean health ministry documented in March that a handful of people had recently died or been sick with Ebola-like symptoms across the border in Sierra Leone. But information about two of those possible infections never reached senior health officials and the team investigating suspected cases in Sierra Leone.

As a result, it was not until late May, after more than two months of unchecked contagion, that Sierra Leone recorded its first confirmed cases. The chain of illnesses and deaths links those cases directly to the two cases that were never followed up in March. Sierra Leone has since tallied about 9,400 reported Ebola infections, more than any other country. The same missed cases are linked to Liberia’s vast second-wave outbreak, identified in late May, with almost 8,000 reported infections to date.

The leaders of the initial response agree that they did not deploy nearly enough people to the region, and that they withdrew too soon. There was managerial confusion in the WHO, which was already stretched by budget cuts and competing demands. Some in the WHO along with Guinean officials played down the threat, leading to overconfidence and inattention. Other international and nongovernment groups devised public-education campaigns that in some instances did more harm than good.

Dr. Peter Piot, who helped discover Ebola in 1976, and Jeremy Farrar, a British infectious disease specialist, called the West Africa outbreak "an avoidable crisis" in an editorial published online in September in The New England Journal of Medicine. In the same issue, WHO officials said of the March to July period that "modest further intervention efforts at that point could have achieved control."

In some of the worst luck in epidemiological history, this outbreak occurred at the bustling intersection of three of the world’s poorest and least developed countries. Doctors in the region were rarer than paved roads and clinics and hospitals, where they existed, often lacked essentials like running water, hand soap and gloves.

International health groups had largely pulled out of West Africa during the civil wars that devastated Liberia and Sierra Leone during the 1990s. When the Ebola outbreak began, the CDC’s staff in the region consisted of one malaria researcher in Guinea.

Complicating matters, the same ethnic group — the Kissi — inhabited the forested region across all three nations, and extended families moved easily on foot and by dugout canoe across a pinwheel of disregarded national borders.

Governments attempted to broadcast the message that Ebola was spread through contact with vomit, feces and blood, and that bodies remained highly contagious after death. But communities often continued to wash the bodies of the dead, a step considered essential to a dignified burial and a contented afterlife. The arrival of moon-suited health workers in convoys of white trucks, armed with chlorine sprayers and thermometers, bred resistance and secrecy.

"Old disease in new context will bring you surprises," Dr. Margaret Chan, the director-general of the WHO, said in an interview in December in her office in Geneva.

On conference calls before leaving Guinea last May, Rollin advised his supervisors in Atlanta that the situation seemed stable enough that the CDC could probably pull out after another month.

But not long after his return, Rollin noticed disturbing trends in the reports landing in his inbox. First, an uptick in cases in southeastern Guinea. Then the first confirmed infections in neighboring Sierra Leone. Then a death at a hospital in Monrovia, Liberia’s capital, and the first case in Conakry in a month.

After a deceptive lull, the virus was back, with ruthless force.

"Damn," the old Ebola hand thought, "we missed it."

The bad news then came like a fusillade. By June 21, Doctors Without Borders had pronounced the epidemic "out of control."

Yet the WHO waited until Aug. 8 to declare the epidemic "a public health emergency of international concern," its top threat level. That was two weeks after two American aid workers were infected in Liberia and a man sick with Ebola flew from Liberia to Nigeria.

By September, the WHO’s Geneva leadership and the United Nations had asserted control over the response.

Today, even as infection rates are starting to decline in some areas, there can be more reports of new cases in just two days than were recorded in the first two months of the outbreak.

Even as they continue to battle Ebola across West Africa, the virus hunters find themselves soul-searching about how many lives might have been saved had there been a bigger, more effective initial response. If the epidemic in West Africa has demonstrated anything, it is that a foe as remorseless as Ebola must be met with a killer instinct that is just as unrelenting.

"There is no room for optimism as long as you are dealing with an Ebola virus," said Dr. Bruce Aylward, who now leads the WHO response. "It’s not about low numbers. It’s about zero. We have got to get to zero."

Rollin accepts a share of responsibility. He and other leaders should have recognized how distinct West African culture was, he said. He should have better appreciated how lax the tracing had been, and that the virus’ disappearance from view did not mean that it was gone.

But he also argues that scientists can act only on the facts as they know them, and that much of what happened in West Africa could not have been foreseen, at least not in the fog of an emerging crisis.

"It was an unprecedented outbreak; it never happened before," he said. "There were a lot of things we didn’t know at that time. No one could have imagined that it would be what we have now."

As the death counts rose, the CDC sent Rollin back to Guinea in mid-June. The beds that sat empty in May were now filling. The hopeful mood of a month before had given way to defeat. As he looked at the rampaging caseloads across the border in Liberia and Sierra Leone, he could tell those countries were too overwhelmed to track the chain of transmission.

"They were just counting the dead," he said.

Before year’s end, Rollin would be dispatched to Liberia, by then flooded with Ebola cases; to Dallas, where a Liberian man, Thomas Eric Duncan, was deathly ill with the virus; to New York, where an American, Dr. Craig Spencer, developed symptoms after returning from Guinea; and to Mali, which reported its first cases this fall.

It would be awhile before he would get a full night’s sleep.

© 2014 The New York Times Company