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.”