If the Ebola epidemic continues to surge in the three worst-affected countries — Guinea, Liberia and Sierra Leone — its economic impact could grow eight fold next year, dealing a potentially catastrophic blow to the already fragile states, according to a World Bank analysis released Wednesday.

The largest economic effects of the crisis are from the “fear factor” — people’s concerns about contagion, officials said.

“Already, the fear factor from the Ebola outbreak has reduced labor-force participation, closed places of employment, disrupted transportation, and motivated some government and private decision-makers to close airports and seaports,” said Jim Yong Kim, president of the World Bank Group.

In a separate development, the international aid group Doctors Without Borders announced Wednesday that a staff member, a French nurse, has been infected with the virus while working at the area’s largest facility, which is outside Monrovia, Liberia’s capital. She is the first foreign medical staffer for the humanitarian organization to develop the disease, the group said.

The nurse was put in isolation Tuesday after developing a fever. Laboratory tests confirmed she had contracted Ebola, and she will be taken to France for treatment.

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An investigation to determine how the woman became infected shut down the treatment center to new patients Wednesday as the disease continued to spread through the city of 1.5 million people.

The treatment center in Paynesville, Liberia, has more than 160 beds and a staff of 486 Liberians and 55 foreign nationals. Doctors Without Borders is continuing to care for 110 people inside the facility, said Laurence Sailly, the group’s emergency coordinator.

Health-care workers have been among those hardest hit by the Ebola epidemic in West Africa, but Doctors Without Borders, which has been on the front lines since March, had not had a foreign national working in any of its treatment centers get infected.

Seven West Africans working in the group’s treatment units have come down with the virus, and three have died. The organization has about 200 international and 1,700 national staffers fighting the deadly hemorrhagic disease in West Africa.

“MSF applies very strict protocols of protection for its staff — before, during and after their time in a country affected by the current Ebola outbreak,” Brice de le Vingne, its director of operations, said in a statement. “This dramatically reduces the risk of transmission of the disease. However, the risk is part of such an intervention, and sadly our teams are not spared.”

The group’s strict protocols have been widely praised by global health officials. Its experts are among several health officials who are working with the World Bank to develop a clinical protocol that all facilities would use to treat and prevent infections. Other experts include Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health; Paul Farmer, an infectious-disease specialist at Harvard University; and officials from the World Health Organization.

The protocol is expected to be completed in the next week or two, Kim said.

The Ebola epidemic over time

World Bank officials said that if the deadly virus is not slowed in the three most affected countries, there could be broader regional contagion, especially through tourism and trade.

The tourist season begins in November, and Senegal and Gambia in West Africa welcome many tourists, economists said. Depending on the scenario, the spread to other African countries could cost “potentially many billions of dollars,” he said.

The analysis said the outbreak has taken a toll on the economies of those three countries. The bank predicts that the combined cost to be $359 million this year, a reduction in growth ranging from 2.1 to 3.4 percentage points, a huge portion of the tiny economies of these countries.

But the analysis found that economic costs can be limited if swift national and international responses occur in the next four to six months, and on a large-enough scale, Kim said.

In that scenario, the economic costs could be $97 million by the end of 2015. If the virus continues to increase exponentially, as World Health Organization officials have said, then the economic impact on the three worst-affected countries could be as high as $809 million by the end of next year.

The virus’s spread to other African countries could cost billions of dollars, Kim said, “and depending on the scenario, we think potentially many billions of dollars.”

Kim said he was encouraged by President Obama’s plan to significantly increase U.S. resources devoted to fighting the epidemic.

“Time is of the essence,” he said. “We need a comprehensive response on the ground to prevent this eight-fold impact next year.”

The service sector has been hardest hit in the affected countries. Mining, especially in Liberia, is chiefly responsible for the disruption, economists said.

The World Bank on Tuesday approved a $105 million emergency grant to the three countries most affected by the epidemic. Liberia will receive nearly $52 million, Sierra Leone will receive $28 million, and Guinea will get $25 million.

The money, part of a $230 million World Bank package to the countries, will go toward essential supplies and drugs; protective equipment; hazard pay and death benefits for Ebola health workers and volunteers; and contact tracing, among other things.

Given the latest estimate from the WHO that the international community needed to provide at least $1 billion in additional assistance to keep the number of infections “within the tens of thousands,” Kim said he would be open to securing more World Bank funding.

He said the board approval for the current emergency grant was unanimous, which is rare. “The board members told me if we need more money to go back to them,” Kim said.

Bernstein reported from Monrovia, Liberia.