Abbas Dulleh , Associated Press
A health worker is sprayed with disinfectant after he worked with patients that contracted the Ebola virus, at a clinic in Monrovia, Liberia, Monday, Sept. 8, 2014. 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 U.N. group was infected with the dreaded disease last month, a spokeswoman said Tuesday.

LONDON — World Health Organization staffers battling an Ebola outbreak in Sierra Leone now have better working conditions — including larger, more private quarters — after one of their scientists was infected with the deadly virus.

The news came Tuesday as another American sickened by Ebola landed in Atlanta to get treatment at Emory University Hospital.

The Ebola outbreak sweeping West Africa is thought to have killed more than 2,000 people and has taken a particularly high toll on health workers. After a Senegalese epidemiologist with WHO tested positive for the disease, the agency conducted an investigation into how he became infected. A second WHO health care worker, a doctor, has also been infected in Sierra Leone.

While the agency 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 workers no longer share living space with other agencies.

Changes were also made to working procedures, including more temperature checks for everyone coming into the WHO office and living quarters, Alexander said.

She said the investigation report was "pretty clear" about how the epidemiologist was infected.

"It's not a new or unexpected risk," she said. Epidemiologists do not treat patients but are sometimes involved in tracing those in contact with Ebola patients or working with safe burial teams for Ebola victims.

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

"If (protective equipment) is worn properly, there is no risk of infection," said Dr. David Heymann, a professor at London's School of Hygiene and Tropical Medicine. "Unfortunately, in the case of Ebola, a mistake can be fatal."

Doctors Without Borders, which runs most of the Ebola clinics in West Africa, has never recorded a single infection among its staff. Ebola is spread by contact with bodily fluids such as blood, vomit, urine or diarrhea and in this outbreak has killed roughly half of those known to be infected.

Other experts said the safety of health workers treating Ebola patients must be continually assessed and noted that not all protective equipment was equal.

"A glove is not just a glove," said Michael Osterholm, a professor at the University of Minnesota who advises the U.S. government on infectious diseases. "There are real questions here about what equipment is being used and how well it's protecting health workers."

He said while the infections of two WHO staffers didn't yet constitute a pattern, the cases were worrying.

"If we see more numbers getting infected, we will have to ask serious questions about whether there were any lapses," he said.

Of the more than 3,500 people believed to be infected with Ebola in Guinea, Liberia, Sierra Leone, Nigeria and Senegal, about 250 have been health care workers. The hardest-hit countries already had too few doctors and nurses before the outbreak.

Most of those staffing Ebola treatment centers are locals and experts say several hundred more foreign health care workers are needed.

The African Union promised Monday to send at least 100 doctors, nurses and personnel to Liberia, Sierra Leone and Guinea for six months, funded by $10 million from the U.S. Agency for International Development and $6.5 million from the European Union. It was not clear when the first teams would arrive.

The United States and Britain also announced Monday they will build new Ebola 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, with a special section for health workers.

Associated Press journalists Sarah DiLorenzo in Dakar, Senegal, and Elias Meseret in Addis Ababa, Ethiopia, contributed to this report.