The World Health Organization has changed how its staff are responding to an Ebola outbreak in Sierra Leone after a scientist working with the agency was infected with the disease last month, a spokeswoman said Tuesday.

A second WHO health care worker, a doctor, has now been infected, the U.N. 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 on Tuesday to receive an American infected with Ebola while working in West Africa.

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" in revealing how the 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.

Alexander said a second investigation has now begun into how the WHO doctor was infected, and the agency has also recommended the Sierra Leone government stop accepting new patients into the Ebola clinic where the incident occurred.

Of the more than 3,500 people believed to be infected in Guinea, Liberia, Sierra Leone, Nigeria and Senegal, about 10 percent have been health care workers. The hardest-hit countries already had too few doctors and nurses before the outbreak, and the toll has only made it harder to respond to the crisis and recruit more caretakers.

While most of the people staffing treatment centers are locals, experts say several hundred more foreign health care workers are still needed. The African Union promised Monday to send at least 100 people to Liberia, Sierra Leone and Guinea for a six-month medical support mission. The European Union will give 5 million euros ($6.4 million) to fund the mission. It was not clear when the first teams would arrive.

The United States and Britain also announced Monday that they will build new treatment centers in West Africa. The American one, in Liberia, will be exclusively for treating health care workers. Britain is also sending military engineers and medical staff to run its clinic in Sierra Leone that will include a section to provide high-quality specialized care for health workers.