Health officials screen a women with a portable remote thermometer for high temperature, an Ebola symptom, in the city of Port Harcourt, Nigeria, on Sept. 2, 2014. (Peace Udugba/EPA)

The Nigerian Ministry of Health announced on Monday that 477 people are under surveillance for Ebola in the oil city of Port Harcourt.

Until recently the outbreak in Nigeria was almost completely confined to Lagos, where the first case of Ebola was brought by a Liberian-American, Patrick Sawyer, who died days after arriving in the airport in July.

All the 477 were involved in a chain of contacts that demonstrated how viciously the virus can spread. Before he was hospitalized, Sawyer came in contact with someone. That person traveled to Port Harcourt, where he sought medical treatment from a doctor. The doctor later died from Ebola, requiring authorities to track down everyone with whom he had been in contact.

The worst outbreak of the virus on record has killed more than 2,000 people across Liberia, Guinea, Sierra Leone and Nigeria, and spread to Senegal. The World Health Organization estimated that 20,000 people could be infected before the crisis ends.

As of Monday there were 21 cases of Ebola in Nigeria — 19 confirmed, 1 probable and 1 suspected — according to the latest WHO figures, Daniel Epstein, a spokesman for the organisation, confirmed.

While foreign military are being called on to combat the virus in the worst affected countries, on a recent visit to the Ebola-hit region the U.N. System Coordinator for Ebola, David Nabarro, praised Nigeria’s containment of the virus.

But the hundreds of potential cases in Port Harcourt show just how easily an outbreak can spread.

One of the first cases monitored, a regional official who had direct contact with Sawyer in Lagos, managed to evade surveillance in July and travel to Port Harcourt, a city at the center of Africa’s oil industry. There he sought treatment from a local doctor.

In August, that Port Harcourt doctor died of Ebola, stoking fears of a new outbreak in the country of more than 170 million. Less than a week after the doctor’s death, more than 70 people were under surveillance, but now that net has widened to more than 400. The doctor’s wife is among those who have contracted the disease.

A boy has his temperature taken with an infrared thermometer prior to entering the UJ Esuene Stadium in Calabar to watch the 2015 African Cup of Nations qualifying football match between Nigeria and Congo on September 6, 2014. The deadly virus has claimed 2,097 lives out of 3,944 people infected in Liberia, Guinea and Sierra Leone, since emerging last December, the UN's health organ said after a two-day crisis meeting in Geneva. Eighteen people have so far been confirmed to have Ebola in Nigeria and seven people have died. AFP PHOTO / FLORIAN PLAUCHERFLORIAN PLAUCHER/AFP/Getty Images A boy has his temperature taken with an infrared thermometer in Nigeria on Sept. 6, 2014. (FLORIAN PLAUCHER/AFP/Getty Images)

The new numbers in Port Harcourt are a new challenge, but Faisal Shuaib, head of the country’s Ebola Emergency Operation Center, said the response teams have scaled up their efforts and are monitoring each case.

Shuaib directs an army of contact tracing and surveillance teams made up of more than 500 health workers and volunteers.

They are pinning their hopes on recent gains in Lagos, where the total of 339 contacts originally under surveillance was reduced to 27 as of Monday. Only one single case of Ebola, the Port Harcourt doctor’s widow, currently remains in isolation.

Surveillance requires a mammoth logistical effort. Every day, members of response teams visit each of the hundreds under surveillance. They go to their homes, take their temperatures and check them for symptoms.

As the worst Ebola outbreak in history unfolds in West Africa, The Post's Joel Achenbach explains how the deadly virus wreaks havoc on the human body. (Davin Coburn/The Washington Post)

If the case managers notice a change in temperature, they report it through an alert system to the command center, which deploys a team to evaluate the individual and, if necessary, evacuate the person to an isolation facility.

There’s always a risk of someone evading surveillance. But Shuaib believes there’s more trust of authorities in Nigeria than in some countries hit by Ebola, where quarantines have caused panic and anger. One of the reasons for that trust, he said, is the fact Nigeria has not imposed quarantines or restricted travel.

“In the beginning, the challenge was always a lack of understanding of the disease — people were a little apprehensive of the contact teams coming to their door asking about their contact” with potential Ebola carriers, he explained. “People were scared by the high level of mortality, but with people surviving now, I think that impression has been changed.”

A massive campaign in collaboration with local media has helped raised awareness and daily briefings by the Health Minister have boosted the trust of the public and made work for the surveillance teams easier, Shuaib claimed.

“People even call the contact tracing teams and say: I suspect someone may have symptoms of Ebola — please send a team,” said Shuaib. “It’s really a question of trust,” he added, emphasizing that stories of people surviving encourages others to seek treatment.

For two weeks in Lagos, there have been no new cases of Ebola, he said. But even with no new cases, surveillance teams need to continue their work.

“We are cautiously optimistic, but we don’t by any stretch of the imagination think we are out of the woods,” Shuaib said.

Correction: An earlier version of this story gave the wrong number for Nigeria’s population. It’s more than 17o million.