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Ever since Ebola first appeared in Nigeria in July, health officials have been holding their breath. Could the country keep the virus from spreading outside its capital of Lagos?

The answer is no. The lethal virus has jumped to another major city. The outcome of a public health drama unfolding in Nigeria's oil-producing center could determine whether the world's largest Ebola outbreak is brought under control or spreads throughout West Africa and beyond.

Nigerian officials are scrambling to identify and monitor the health of more than 200 people in Port Harcourt, an industrial city of 1.4 million about 300 miles southeast of Lagos.

"This is a really bad situation," says John Woodall, a London-based epidemiologist who is monitoring the situation for ProMED-mail, a global disease-tracking group. "Oil is the hub of Nigeria's economy, and Port Harcourt is a city where many Nigerians and foreigners mix."

How did Ebola hop from Lagos to Port Harcourt?

It all started at the beginning of August, when a diplomat in Lagos violated a quarantine order and fled to Port Harcourt. That man infected a doctor at the port city, who then had contact with more than 200 people, the World Health Organization said Wednesday. About 60 people had what WHO calls "high-risk exposure" — they were in direct contact with the doctor or his bodily fluids.

The doctor secretly treated the diplomat in a Port Harcourt hotel room. The diplomat reportedly has survived.

The doctor developed symptoms — and thus became contagious to others — on Aug. 11. But for the next two days, he continued to treat patients in his private clinic, performing surgery on two.

As his Ebola symptoms worsened, but before he went into the hospital, the doctor had "numerous contacts" with relatives and friends who came to his home to celebrate the birth of a baby, the WHO said.

After he was hospitalized, the doctor was treated by the majority of the staff at the hospital's clinic over a six-day period, plus doctors at an outside ultrasound clinic. He also had contact with many members of his church, who visited to perform a healing ritual "said to involve the laying-on of hands," the WHO reports.

The doctor died on Aug. 22. His wife got Ebola but has survived. On Thursday the Nigerian Federal Ministry of Health reported that the doctor's sister has Ebola.

The WHO currently reports 21 cases of Ebola and seven deaths in Nigeria. According to Nigerian Health Minister Onyebuchi Chukwu, five of these cases — two of whom have died — are in the Port Harcourt cluster.

But WHO officials are very worried that Nigeria could see many more cases. "Given these multiple high-risk exposure opportunities, the outbreak of Ebola virus disease in Port Harcourt has the potential to grow larger and spread faster than the one in Lagos," the WHO said.

Nigeria is Africa's most populous and richest country. Its public health system is reportedly much more robust than the systems in Guinea, Sierra Leone and Liberia — the three countries to the west where Ebola emerged in March and still spreads out of control.

And Nigeria, by some reports, is doing a good job. Some health officials have praised the government's response to Ebola since the disease was introduced into the country in late July by a Liberian-American businessman who fled to Lagos for treatment.

"I have to say (though I usually find very little to praise in any government in Nigeria) that the speed and efficiency with which the Lagos State authorities reacted was exemplary," Nigerian author Adewale Maja-Pearce writes Friday in The New York Times.

Nigeria has fielded 21 teams to trace contacts of Ebola cases, as well as a burial team and two teams to decontaminate potentially infected surfaces. The WHO has 15 technical experts in Nigeria.

But Woodall is not optimistic about the Nigerians' ability to contain Ebola as it spreads beyond Lagos.

Pointing to the experience in other afflicted countries, he predicts people will be hard to keep under surveillance and quarantine — crucial measures to keep the virus from spreading exponentially.

"They claim they've contacted 96 percent of primary and secondary contacts of the dead doctor, but that doesn't mean those people will stay still to be quarantined," Woodall says. "Allegedly 50 people in contact with the dead doctor have gone into a neighboring state [inside Nigeria]."

"Some will run away," he says. "They don't want to be quarantined. They're scared of being quarantined. So I'm afraid it's going to spread."

Epidemiologist John Brownstein of Harvard University directs a global disease-tracking service called HealthMap. He also worries that Ebola will outpace Nigeria's ability to contain it.

"It's like fighting a wildfire when the wind picks up," he says. "When embers start hopping to new places, you have to redistribute your resources when you'd prefer to focus all of them on a single front."

A further spread of Ebola in Nigeria would carry serious international implications because the nation is an important transportation hub for Africa. Thousands of people pass through Lagos' airport each day on their way to destinations around the world.

An analysis published this week noted that 3,000 to 6,000 travelers fly from Nigeria to the U.S. every week. Thousands more fly from Nigeria to the U.K., Canada, France, Germany, the United Arab Emirates and China, among other countries.

Since people can harbor Ebola virus for one to three weeks before they show symptoms and become contagious, infected travelers cannot always be screened before they disperse.

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