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Health care workers have more than 100 times the risk of catching Ebola in Sierra Leone as the general public there does, according to a new report.

And it’s not necessarily down to failed protective measures in hospitals. Health care workers form their own community, and when one gets sick or dies, he or she can infect fellow medics, the report finds.

The World Health Organization has been saying that health care workers such as doctors and nurses are at special risk of Ebola. It says 622 health-care workers have been infected and 346 of them have died in all the affected countries.

“They can ill afford to lose health care workers."

Sierra Leone already has far too few health care workers — just about 2,400 for a country of 6 million people.

“They can ill afford to lose health care workers,” said Dr. Peter Kilmarx of the U.S. Centers for Disease Control and Prevention, who led an investigation into the high infection rate in Sierra Leone.

Ten physicians have died of Ebola in the current epidemic, including Dr. Martin Salia, who died after being evacuated to the University of Nebraska for emergency treatment last month.

Salia wasn’t even treating Ebola patients. He was a primary care doctor at a Methodist hospital, probably infected, experts believe, when he was treating a patient for other symptoms without suspecting he or she had the virus.

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It’s a story that CDC experts found over and over in their investigation of health care worker deaths.

“We think of health care worker infections as a failure of personal protective equipment,” Kilmarx told NBC News. “But there are so many different ways that they are exposed there.”

For one thing, health care workers tend to socialize with other health care workers. “If one of them gets ill and dies, the other health care workers attend the funeral,” Kilmarx told NBC News. “It’s sort of a multiplier effect.”

And they quietly help their neighbors, providing care to friends and family afraid to go to hospitals. Perhaps the biggest risk is the lack of Ebola testing or what’s called a false negative — when someone’s infected but the test doesn’t show it. The health care workers treat them without taking precautions and when the patients later develop symptoms and become infectious, the caregiver can be infected, too.

“This happened in Kenema with an infant that tested negative and the workers had ongoing exposure and the infant developed Ebola and infected the health care workers,” Kilmarx said. That case has been well documented, when nurses caring for the baby almost all got infected and died.

The obvious solution would be to test patients more than once, but the tests are scarce and workers are already stretched too thin.

The situation’s so bad that Sierra Leone's Junior Doctors Association, which represents doctors in training who are called residents in the U.S., called a partial strike Tuesday. They’re upset that they are not getting the care they should be when infected on the job.

Dr. Jeredine George, president of the group, told Reuters that local doctors were dying at an "alarming rate."

In neighboring Liberia, U.S. troops have built a special Ebola treatment unit for health care workers, to encourage them to stay on the job.

Ebola’s infected more than 17,000 people in Sierra Leone, Liberia and Guinea, according to WHO, and the epidemic’s worsening in Sierra Leone. WHO experts now doubt they can get it under control before the middle of next year, and even then only if countries step up their aid efforts.

Kilmarx and colleagues took national estimates of the health care worker population — that’s where they got the 2,400 number for all of Sierra Leone. They divided this by the number of health care workers infected with Ebola to get an infection rate of 8,285 people per 100,000 people. That’s more than 100 times the infection rate for the country’s population as a whole, 80 per 100,000.

"Less well-controlled settings might inadvertently act to propagate rather than interrupt transmission."

His team questioned surviving health care workers and found 30 percent had been in contact with the body of someone who had died of Ebola. The bodies of people who have just died of Ebola are extremely infectious, and WHO is trying to get all the affected areas to follow strict burial practices.

Another 13 percent had attended funerals. And in the hospitals, the team found a range of failures, including a lack of clear procedures and responsibilities; staff shortages; and inadequate separation of Ebola patients from everyone else. They also found a lack of vehicles to safety transport patients and bodies; a lack of both protective gear and facilities to wash hands and disinfect equipment; poor training; poor waste disposal and unsafe burial of corpses.

"New, high-quality, dedicated Ebola treatment units are being established by international partners in Sierra Leone," Kilmarx and colleagues wrote in their report. But they're not enough, and Sierra Leone's government is trying to step up as well.

"Health authorities must be vigilant in implementation of strict infection prevention and control measures in all healthcare settings and alert to the possibility that less well-controlled settings might inadvertently act to propagate rather than interrupt transmission," they concluded.