MBANDAKA, Congo - The two brothers of Adalbert Wanza made a 170-mile round trip voyage on foot, boat and motorcycle through Congo's dense rain forest to bring him, and ultimately the rest of the world, grave news.

A visitor had died while in their home village, Ikoko-Impenge. A local priest buried him with ritual honors, including giving the corpse its last food and drink. Days later, the priest and most of his family were dead, too. When he heard the story, Wanza, the Catholic bishop of Mbandaka, knew it could be Ebola, which is endemic in the forest, and contacted health officials. Lab tests would prove him right.

Over the next three weeks, the most serious outbreak of the virus since a devastating epidemic in West Africa between 2014 and 2016 spread from deep within the rain forest to bigger towns and finally to Mbandaka, a regional hub of nearly 1.2 million people at the confluence of the Congo and Ruki rivers.

As of Friday, 37 cases of Ebola had been confirmed, four of them in Mbandaka, where it can more easily spread. Twelve confirmed cases resulted in death, as have an additional 13 suspected or probable cases.

The outbreak has triggered a massive effort to contain it, led by the Congolese government and the World Health Organization, and aided by numerous aid groups. Dozens of health workers are bringing experience from the West African epidemic.

They are also bringing an abundance of caution. The WHO, aid groups and West African governments underestimated the last big outbreak, which started in Guinea in 2014. By the time resources were mobilized, Ebola was already in or on its way to the capital cities of Guinea, Sierra Leone and Liberia.

A sense of foreboding hangs over conversations among these groups, even as their work appears to have stemmed the outbreak's growth for the time being. The last confirmed case in Mbandaka was reported May 22, but the horror of the past is still raw - more than 11,000 died in the West Africa epidemic, and cases spread as far as Nigeria's capital, Abuja, and Dallas in the United States.

"We must make sure we leave not even one stone unturned," said Pierre Rollin, an Ebola expert for the Centers for Disease Control and Prevention who is now in Congo. "Signs point to a diminishing number of cases, but there are no guarantees until we have gone weeks without any."

The aid community has gone into overdrive since the Wanza brothers' revelation in early May, which helped alert the national government to the outbreak. Medical organizations are using every strategy available, including an experimental vaccine cleared by the WHO for "compassionate use." It was administered in the waning days of the West African epidemic, but this is its first deployment in the critical early stages of an outbreak. Hundreds have received the vaccine and there are supplies for thousands more.

Vigilance is crucial. The virus is not one of the most contagious - it is transmittable through direct contact such as touching bodily fluids - but symptoms can take days if not more than a week to show, meaning infected individuals can travel long distances and encounter many people before there's any inkling of their condition.

"The outbreak is at a critical point where good contact tracing is going to determine its extent," said Anne Rimoin, a researcher at UCLA who has worked with the Congolese government for 16 years on Ebola.

Contact tracing is the process through which everyone who has interacted with someone suspected of having Ebola is identified and monitored. To succeed, it requires the near-instantaneous creation of a network of informants who surveil movements of people in and out of areas where the virus is suspected.

A tip from one of these people-watchers is what recently led a team of 16 Guinean doctors to a riverside village on Mbandaka's outskirts. A woman had arrived in the village to stay with relatives after her husband died in an area near Ikoko-Impenge, where the outbreak seems to have begun.

Within a day, the doctors had set up a contact-tracing operation. But monitoring contacts isn't the only tool they have: The Guineans were flown in because they are the only ones with hands-on experience administering the experimental Ebola vaccine, during the end of the epidemic in their home country.

Because the vaccine is as-yet unlicensed, everyone who receives it must sign a consent form. Aid workers say that sometimes involves considerable persuasion.

"The lesson from West Africa is this: The concept of a vaccine is not something that someone in rural Guinea or Congo will understand in just two minutes," said Fatoumata Battouly Diallo, 31, the team leader of the Guinean vaccinators.

They try to communicate the danger in a way people can understand. "We say, 'You want to protect your family, right? You want to protect your people? Your signature and five seconds of pain is all that is required.' "

This part of Congo, Equateur province, is the most neglected in a country that generally has little in the way of infrastructure. That underdevelopment complicates the response to Ebola, forcing aid workers to use scarce helicopters or motorcycles to reach affected areas. Roads are so bad it can take a full day to drive 100 miles in a truck.

The vaccines are particularly difficult to transport. They must be kept at a temperature of 76 degrees below to 112 degrees below and have been shipped from Geneva in special containers that can retain that temperature for up to six days.

Ironically, the region's underdevelopment has helped slow the outbreak, officials say. Compared with the parts of West Africa where the last epidemic took place, people in Equateur are poorer and travel less. Whereas Sierra Leoneans infected with Ebola were lured to bigger towns by better health facilities, Mbandaka doesn't have that same draw. And transport itself is more difficult in the rain forest. Most of it takes place on boats - from smoke-belching steamers to cargo-bloated barges to slender pirogues carved by ax out of massive trees.

Although Congo may sorely lack infrastructure, it has one of the most successful Ebola response mechanisms in the world, according to researchers and aid workers. Congo is where Ebola is most prevalent, and the current outbreak is the country's ninth since 1976.

"It has been just amazing to watch how quickly the Congolese health ministry detects these outbreaks, even when they are in such remote areas," said Rollin, of the CDC. The health ministry's close monitoring of unusual or clustered deaths in the rain forest is credited with early outbreak detection, but in many cases the alarm bells are rung by locals, like Bishop Wanza's brothers.

Not everyone in Equateur is taking the outbreak seriously, however. About half of the dozen Mbandaka residents The Washington Post interviewed along the city's one fully paved road said Ebola was a conspiracy. Before the outbreak, foreigners were exceedingly rare in Mbandaka, and the recent influx of hundreds of aid workers has brought large amounts of cash into the city. That has caused many to believe that Westerners brought Ebola here to make money.

Upon seeing foreign aid workers passing by in their Toyota Land Cruisers, some locals have taken to shouting, "Mundele Ebola!" which means "white person's Ebola" in Lingala, Congo's lingua franca. These theories have complicated the process of getting consent for vaccinations in some cases, though most people have ultimately agreed.

"The community in Mbandaka is divided between those who are fearful and those who don't care one bit," Wanza said. "Some people are not aware of how serious this is."

The process of contact tracing can also alienate Congolese who are unfamiliar with Ebola. Suddenly, they are surrounded by strangers, sometimes including people speaking in foreign languages, asking them about every move they've made in the past few days. Their family compound is abuzz with disease detectives, who may be wearing protective gear, or pointing gun-shaped thermometers at their foreheads.

"It is a fact that foreigners are a cause of stress for people here," said Prudence Toukou, a psychologist working for the Congolese health ministry. "Their lives are suddenly invaded and it is hard to understand who these people are."