A surge in Ebola infections in Liberia is driving a spiraling outbreak in West Africa that is increasingly putting health workers at risk as they struggle to treat an overwhelming number of patients.

A higher proportion of health workers has been infected in this outbreak than in any previous one. The latest infection was of a doctor with the World Health Organization treating patients in Sierra Leone. The organization gave no details, but an American who became infected while working in West Africa landed in the U.S. Tuesday to get treatment at Emory University Hospital.

This is the second WHO staffer to be infected in Sierra Leone, and the U.N. health agency said Tuesday that after an investigation of the first case, staffers battling Ebola there now have better working conditions - including larger, more private quarters.

The outbreak sweeping West Africa is thought to have killed more than 2,200 people, and public health experts agree that it is out of control. More than 4,200 people have believed to have been sickened in Guinea, Liberia, Sierra Leone, Nigeria and Senegal.

The disease is spreading particularly quickly in Liberia, where new WHO figures showed that more than 500 new cases were recorded in a week. The organization warned Monday that it expects thousands of new cases in the country in the coming weeks. Each time a new treatment center is opened there, it said, it becomes immediately overwhelmed, "pointing to a large but previously invisible caseload."

A shortage of doctors and nurses to care for these patients is being exacerbated by the sheer number of health workers becoming infected. But that shortage may also be the reason why they are getting infected, experts say.

"The fact that people that are highly trained are getting infected is because the number of cases is bigger than the bed capacity," said Jorge Castilla, an epidemiologist with the European Union's Department for Humanitarian Aid who visited the hardest-hit countries recently. "When you have too many patients, you have too much to do, you get tired and when you're exhausted, you make mistakes."

After a Senegalese epidemiologist with WHO working in Sierra Leone tested positive for Ebola, the agency conducted an investigation into how he became infected.

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 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.

Michael Osterholm, a professor at the University of Minnesota who advises the U.S. government on infectious diseases, 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.

Around 250 health care workers have been infected, a high toll in countries that 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 other 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.