CHARLOTTE, N.C. - Working with Ebola patients in Liberia, American pediatrician Alan Jamison treated as many people as he could as the country slipped into chaos. Each day, more patients showed up at the hospital's doors. The deadly virus wasn't the only danger: Ebola was causing such fear that some Liberians were threatening to burn down the isolation unit with doctors and patients inside.

His medical missionary group pulled him out early as a precaution. Still, the 69-year-old retiree says he'd return.

"This is where the need is," Jamison explains. "This is my calling."

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Jamison isn't alone - even after three American aid workers fell sick, many other doctors, nurses and other health care volunteers are on their way to West Africa, helping to staff hospitals and clinics and screen travelers to slow the epidemic's spread.

Why are so many willing to put themselves in harm's way?

"It's a call, a zeal, a devotion. It's an acceptance of a professional life outside the ordinary, with an element of adventure," said William Schaffner, an infectious disease specialist at Vanderbilt University in Nashville, Tennessee.

Hospital volunteer Nancy Writebol and Dr. Kent Brantly were already in Liberia when the outbreak began, and decided to stay at the charity-run ELWA hospital in Monrovia to help. Richard Sacra, a 15-year ELWA veteran, immediately volunteered to leave his family in suburban Boston and return to the hospital when Writebol and Brantly got sick. Jamison also worked there.

All are committed to their cause. Like Jamison, Nancy Writebol and her husband, David, told The Associated Press that they'd consider going back. Brantly said he couldn't return just yet, but would keep campaigning to end Ebola. Sacra also had no regrets, his wife said as the doctor was evacuated to the isolation unit in a Nebraska hospital.

"Once you go and you see the Lord at work, I mean, there's nothing else that you want to do," Nancy Writebol said.

These volunteers are passionate, but there's also a cold logic to their commitment: This epidemic that has killed more than 2,000 people and sickened 3,900 in five West African nations won't end unless more experienced health care workers confront it directly.

Ebola is being spread by people, in hospitals, homes and funerals. People catch the virus when they have direct contact with the blood or bodily fluids of those who are sick and dying, or already dead. At ELWA, Jamison trained workers how to protect themselves and the wider population.

The hospital in Monrovia is operated by Charlotte-based SIM USA, and includes more than 200 beds as well as the 50-bed isolation unit for Ebola patients.

Keeping those populations separate is essential, Jamison said, but is no simple matter. He trained workers to wear a mask and gloves and screen new patients from several feet away before they were allowed to enter. When patients showed signs of Ebola, a worker wearing a protective suit would be summoned to bring them to a holding area for evaluation and then to the isolation unit if necessary. But the screeners have to ask the right questions to suss out the truth in such a fearful environment, Jamison said.

And if anyone masking Ebola symptoms is allowed inside, they could expose many more people who don't routinely wear full-body protective suits.

"Sometimes I felt safer in the Ebola unit than in the hospital," Jamison said.

Most international aid organizations are quite familiar with the risks of sending health care workers into terrain plagued by war, political turmoil and disease. But this Ebola epidemic has posed serious and unique challenges.

"We're balancing tremendous need in a risky environment," said Joe DiCarlo, a vice-president at Jamison's sponsor, the Portland, Oregon-based Medical Teams International. It's a non-denominational Christian group that has been working in Liberia for 10 years, with 15 permanent medical staff in the country, including three Americans, supporting 240 clinics around the country.

"We are finding volunteers who want to go, even though they are fully aware of the situation," DiCarlo said. "We're getting the people we need, but I can't say we're overwhelmed with requests."

Liberia has the largest caseload and death toll, but many of its hospitals have been closed. There are just two large hospitals still operating in the country of four million people, said George Salloum, SIM's financial officer.

About 250 people work at ELWA - most of them Liberian. They usually have between three and seven American doctors who serve two to three years.

Right now, there's only one American doctor left on-site.

After some airlines stopped flying to countries affected by the outbreak, it's been much harder to get enough medical supplies to keep up with demand, including critical protective gear to keep its doctors and nurses safe. ELWA is no exception. Staffers go through thousands of disposable protective suits a week. Salloum said they recently received a shipment of protective gear, but they're running short of other supplies, including the intravenous fluids and electrolytes needed to keep Ebola victims alive long enough for their bodies' immune systems to fight the virus.

"We just take for granted how easy it is in America to get these things," he said.

The number of patients in the Ebola isolation unit fluctuates - but is usually close to capacity, he said.

With so much need, SIM has no plans to pull out, said Will Elphick, the group's director for Liberia.

"You have to weigh out the risk compared with what you feel you can do with a situation where there is so much need," Elphick said. "We still want to support our Liberian colleagues."

That will require more supplies, and volunteers.

People like Rendi Murphree, an epidemiologist with the Centers for Disease Control and Prevention, who volunteered to leave Tuesday for Liberia, where she will screen passengers leaving the country at the Monrovia airport.

"This is the type of crisis situation that you prepare most of your career for, so it's exciting to help in a big response that has so much impact," said Murphree, who also served in Haiti after its 2010 earthquake. "I haven't had anyone try to dissuade me in earnest ... I've had a lot of feedback from people who say they're proud of me, they can't wait to hear how the deployment goes."

However, "my mother, grandmother and mother-in-law have put me on the prayer list at church, and notified the extended family to do the same," she said.