Although few medical experts realize it, part of the population in West Africa is immune to the Ebola virus, according to virologists who specialize in the disease.

Assuming they are correct, and if those people can be identified, they could be a great help in fighting the outbreak. Immune persons could safely tend the sick and bury the dead just as smallpox survivors did in the centuries before smallpox vaccine.



Also, antibodies could be harvested from their blood to treat new Ebola victims.



But many factors remain unclear, including which Africans have antibodies and how much antibody is needed to be protective. The biggest mystery is how the immunity arose, and there is a mix of explanations, like silent infections and fruit contaminated with bat saliva.



“It's fair to say that some people are immune,” said Robert F. Garry Jr., a Tulane University expert in hemorrhagic fevers who works in Sierra Leone. “But we don't know if it's 1 percent or 2 percent or 20 percent.”



Right now, there are about 1,800 survivors of the current West African outbreak, all of whom are now immune, of course. But there may be many thousands more.



Small studies of household contacts of Ebola victims show that some people are infected without ever falling ill — perhaps because of some unknown genetic advantage.



But many Africans who have never seen a victim also have antibodies.



It is possible that some get low doses of virus by eating infected monkeys or bats that are undercooked.



“If someone got just two or three or four virus particles, if it enters through the mucus membranes in the mouth, yes, it's plausible,” said Thomas W. Geisbert, a hemorrhagic fever expert at the University of Texas Medical Branch in Galveston. “It would take a while for the virus to get going, and it's a race with the clock. The immune system gets a chance to fight it off.”



Antibodies, Y-shaped proteins that attach to a virus and block it from invading cells, are the immune system's first line of defense; the second line is white blood cells primed to recognize and digest the virus.



One of France's leading Ebola experts says he believes that many rural villagers are “vaccinated” by eating fruit gnawed on by bats and contaminated with their saliva.



“We imagine that this is the main route,” said Dr. Eric M. Leroy, a veterinarian and virologist at the International Center for Medical Research in Franceville, Gabon. “But it is a hypothesis. We do not have the evidence.”



Determining the overall level of immunity in West Africa would require testing thousands of blood samples, an impossible task in the current chaos, especially when any slip of a needle or a broken vial could fatally infect a health worker.



But in 2010, Leroy led such a study in Gabon, a Central African country that had four Ebola outbreaks from 1994 to 2002.



His teams took 4,349 blood samples in 220 randomly selected villages. They found that 15 percent of Gabon's population had antibodies. But it varied widely: near the coast, only 3 percent did; in some jungle villages near the Congo border, up to 34 percent did.



Also, their antibody levels varied widely, and what level is protective is roughly known for lab monkeys, but not for humans.



“I don't think we have a good idea of what constitutes a person who's going to survive versus a person who's going to succumb,” said Randal J. Schoepp, head of diagnostics at the U.S. Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland, who led a study of blood from patients in a Sierra Leone hospital who were originally thought to have Lassa fever but did not. Nearly 9 percent had Ebola antibodies — and the samples dated from as far back as 2006, proving that the virus circulated long before this year's outbreak.



Also, there is anecdotal evidence that some West Africans are resistant. Victims have relatives who never get sick. At the funeral of a traditional healer where 14 women became infected, at least 26 other mourners did not, Garry said, even though most probably touched the body.



There is firm evidence for silent infections.



In 2000, Leroy's team studied 24 Gabonese who had tended victims without ever falling ill. Eleven had not just antibodies but remnants of virus and markers of inflammation in their blood — meaning they had clearly been infected but had defeated the virus on their own.



A similar 1999 study by U.S. scientists in the Democratic Republic of Congo found similar results in five of 152 household contacts.



Those who are immune can donate blood containing antibodies to be given to acutely ill patients, as was done for Dr. Kent Brantly, one of the first two Americans to get Ebola. He survived, although his Emory University doctors later said it was unclear whether the transfusion or an experimental drug, ZMapp, containing cloned antibodies, helped him at all.



Having those who are immune be caregivers and body carriers makes sense, said Tom Skinner, a spokesman for the Centers for Disease Control and Prevention.



“But we can't count on their immunity,” he added. “They would still need full personal protective gear.”



Relying on such measures may be inevitable, Garry said, adding: “There's no more ZMapp out there. It's time for creative solutions.”