ATLANTA/WASHINGTON/DAKAR/LONDON: An ambulance transporting the fourth American aid worker sickened with the Ebola virus has arrived at Emory University Hospital.

The patient arrived about 10:20 a.m. Tuesday, walking from the hospital to the ambulance. A medical plane first transported the patient to the Atlanta metro area earlier in the morning.

Compared to those earlier arrivals, the scene at the hospital campus was calm Tuesday. Emory’s previous two patients recovered and have been released. Another patient, a doctor, is being treated in Nebraska.

Hospital officials released no details on the latest patient, but the World Health Organization says a doctor who has been working in an Ebola treatment center in Sierra Leone tested positive for the disease. It said the doctor was being evacuated.

The death toll from the Ebola outbreak, which is spreading across West Africa, has surpassed 2,000, out of nearly 4,000 people who have been infected, according to the World Health Organization.

World Health Organization figures show that a surge in new cases in Liberia has driven up the number of people believed to be infected with Ebola to more than 4,200.

That new toll includes more than 500 news cases in Liberia in just a week. The UN health agency has said that it expects thousands of new infections in Liberia, the hardest hit country in the current outbreak, in the coming weeks.

An Ebola epidemic in West Africa is spiraling out of control and moving faster than efforts to contain it.

New figures published by WHO on Tuesday attribute more than 2,200 deaths to Ebola.

World Health Organization staff battling an Ebola outbreak in Sierra Leone are working in larger quarters and no longer live with people from other agencies, after a scientist with the UN group was infected with the dreaded disease last month, a spokeswoman said Tuesday.

A second WHO health care worker, a doctor, has now also been infected in Sierra Leone, the UN health agency announced Monday. It has not released any more information about the doctor, but Emory University Hospital in the United States has said it is preparing to receive an American infected with Ebola while working in West Africa on Tuesday.

The Ebola outbreak sweeping West Africa has killed more than 2,000 people and has taken a particularly high toll on health care workers. After a Senegalese epidemiologist with WHO tested positive for the disease, the agency conducted an investigation into how he became infected.

While WHO is not releasing the results of the investigation, spokeswoman Nyka Alexander said Tuesday that staff living and working quarters in Sierra Leone have been expanded to make them less cramped and they no longer share living space with officials from other agencies. Changes were also made to working procedures, including more routine temperature checks for everyone coming to the WHO office and living quarters, Alexander said.

She said the investigation report was “pretty clear” about revealing how the Senegalese epidemiologist was infected but said the agency wouldn’t be releasing details.

“It’s not a new or unexpected risk,” she said. Epidemiologists do not treat patients but are sometimes involved in contact tracing to follow up potential cases and liaising with safe burial teams for Ebola victims.