The Ebola epidemic in West Africa is entering a critical and complicated new phase, one that global health officials refer to simply as “getting to zero.” Even as cases continue to plummet, reaching that elusive goal means marshaling more resources than ever to stamp out the virus.

The central challenge remains quickly isolating every Ebola victim, painstakingly tracing all the people who might have been exposed and monitoring them for several weeks to be sure they don’t get sick, a process that experts say could take several months, if not longer. Only then will the thousands of viral transmission chains vanish one by one.

“The last case is the hardest case,” said Tom Frieden, director of the Centers for Disease Control and Prevention. “We won’t get to zero simply by hoping things are going down but by intensively following up on every single possible case. We’ve got to find where the cases came from, trace them back and identify the rivulet of the flood and stanch it.”

To do that, the CDC has continued to ramp up its response in the region. It now has 214 staffers in West Africa, more than at any other time during the outbreak. The agency plans to open new offices in Guinea, Liberia and Sierra Leone in the spring.

All told, the U.S. government is helping to fund the work of more than 10,000 personnel — mostly African nationals — who are battling Ebola. The number is expected to increase in coming weeks. “That footprint is larger than it’s been throughout the response,” said Jeremy Konyndyk, a top official with the U.S. Agency for International Development.

The aid group Doctors Without Borders has increased the number of “rapid response” teams to fight the spread of the virus in far-flung communities. The United Nations has helped fly in testing labs and ambulances.

Soka Moses, a physician at an Ebola Treatment Unit in Monrovia, Liberia, discussed why he thinks Liberia cannot be Ebola-free until the virus is gone from West Africa. (Whitney Leaming/The Washington Post)

The surge of resources comes as many parts of Liberia, Sierra Leone and Guinea are showing flickers of normalcy, with infection rates falling and businesses reopening. In late January, the region reported fewer than 100 new Ebola cases in a week, the lowest such figure since June.

In Liberia, once the epicenter of a crisis that has killed nearly 9,000 people, only a handful of active Ebola cases remain. In Sierra Leone, officials recently lifted travel restrictions that had been in place for months. In Guinea, schools have begun reopening, and the government recently held a raucous ceremony in the capital to launch a campaign to end Ebola over the next 60 days.

Reducing the number of new cases from 1,000 a week to 100 a week was critical to saving lives, preventing Ebola’s spread and breaking the curve of the epidemic, officials said. But getting from 100 cases a week to zero may take longer, even with determination.

Getting to zero is time-consuming. Health officials or volunteers go to the homes of all the people who had direct contact with an Ebola patient, writing down their names and monitoring them daily for 21 days — the incubation period — for symptoms such as fever, vomiting and diarrhea. If any of these people becomes a confirmed Ebola case, he is supposed to be isolated and treated as soon as possible, and the monitoring of his contacts starts all over again.

“You really need to follow the trail, the epidemiological puzzle, to make sure you don’t miss that one person or one family that could start another flash somewhere,” said Henry Gray, operations coordinator for an Ebola task force run by Doctors Without Borders. “Everybody needs to see this through to the end.”

Gray said officials will know they have the outbreak under control when 100 percent of the individuals who test positive for Ebola are already on a watchlist of people potentially exposed to the disease. “We should not be surprised,” he said.

Confirmed weekly Ebola cases reported from Sierra Leone. (World Health Organization/Ebola Situation Report)

For now, officials are still being surprised too often. The most recent data for Sierra Leone shows that 21 percent of confirmed cases occurred among people being monitored by authorities; for Guinea, 54 percent of confirmed or probable Ebola cases happened among registered contacts. For Liberia, all of its seven new cases involved people whom authorities were monitoring.

“As we have seen time and time again, an upsurge in new cases can follow a single unsafe burial or violent act of community resistance. Both of these high-risk situations are still occurring,” World Health Organization Director-General Margaret Chan said at recent gathering of the group’s executive board in Geneva. “We know that not all cases, and especially not all deaths, are being detected and reported.”

Tracking every contact of every victim, especially in a region with spotty infrastructure and porous borders, can sometimes be maddening.

In Monrovia, for instance, a small number of new cases involves following more than 100 contacts for several weeks. Guinea’s forested region, where the Ebola outbreak originated more than a year ago, remains particularly worrisome because many of the villages are isolated. And even when a particular area has no Ebola cases, teams must make sure no new infections occur for 42 days, or two entire incubation periods.

Confirmed weekly Ebola cases reported from Liberia. (World Health Organization/Ebola Situation Report)

Across the region, reliable Internet and cellphone coverage is lacking, and reliable electricity and roads are scarce. Contact tracers must often improvise.

An infected man from Monrovia recently traveled hundreds of miles to a northern part of Guinea — particularly worrisome because the area borders Senegal, which recently reopened crossings with Guinea. A CDC team traveled by helicopter to find him and track down his contacts there and in Monrovia, trying to prevent another flare-up of the disease.

The decline in new Ebola cases, while welcome, has also led to concerns about complacency. In the hardest-hit places, such as Monrovia and Freetown, people are returning to daily routines, and the curfews and no-touching policies that took root during the crisis have begun to ease.

In the Guinean capital, Conakry, which never experienced the massive number of infections that plagued the other capitals, health officials and aid workers remain concerned that Ebola-prevention measures have not been widely embraced. It’s still common for people to shake hands, hug and kiss. On a recent day at the building that houses the nation’s Ebola coordination committee, no one stopped visitors to check them for fever or to make them wash before entering. During the Africa Cup of Nations soccer tournament last month in Conakry, people danced in the streets, holding hands and embracing to celebrate the start of Guinea’s game against the Ivory Coast.

Confirmed weekly Ebola cases reported from Guinea. (World Health Organization/Ebola Situation Report)

In some traditional healer communities, particularly in Guinea — which dwarfs Sierra Leone and Liberia in size and has more people than both of those countries combined — the residents continue to resist efforts to reduce Ebola infections.

Health-care workers also have continued to get infected at worrisome rates, and officials are concerned that doctors and nurses may not be wearing all their protective gear or removing it correctly.

It probably will be summer, and perhaps much later, before the three Ebola-ravaged countries in West Africa can declare themselves free of the virus.

“The only way you stop it and not worry anymore is when the very last person is no longer transmitting — is either dead or better,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. “There’s always the threat of the embers in the fire becoming a flame.”

Amy Brittain in Guinea contributed to this report.