As Ebola spreads out of control in West Africa, the World Health Organization reports a black market in blood from Ebola survivors. The epidemic is killing up to 70 percent of those who get sick, but the thousands who have survived have blood teeming with antibodies that protect them against infection again.

Ebola survivor Dr. Kent Brantly has donated serum to three other patients — fellow medical missionary Dr. Rick Sacra, NBC camera operator Ashoka Mukpo and Dallas nurse Nina Pham. No one knows if it’s helped, but in theory Brantly’s antibodies should have kick-started their immune responses.

Because there’s no specific cure for Ebola, people are asking why not make blood available. Researchers in Texas say there may be an unexpected source: people with silent Ebola immunity.

“There is limited evidence from past outbreaks that suggests there probably are quite a few people who get exposed, who get infected, without ever developing symptoms and without ever developing illness but they develop immunity,” said Lauren Ancel Meyers, a professor in the Department of Integrative Biology at The University of Texas at Austin.

In other words, Ebola might be acting like its own vaccine.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Meyers hasn’t tested any blood herself, but she and her team went through published reports about studies done on Ebola patients. In some outbreaks, up to half of people at high risk of infection who were watched never got sick, but they had antibodies against Ebola in their blood.

“If this turns out to be true, there is widespread silent Ebola infection that is immunizing.”

The best evidence of this happening with Ebola was a study done during a 1996 outbreak in Gabon, when researchers monitored 24 contacts of known Ebola patients who never became sick. Eleven of them had antibodies to Ebola.

“If this turns out to be true, there is widespread silent Ebola infection that is immunizing,” Meyers said. “We should really investigate it further.”

It’s not unheard of. Immunologists know that people can get exposed to a virus and never become ill. One study showed that people in Peru appeared to have survived rabies — a virus believed to be virtually 100 percent fatal. One in 10 people tested had antibodies to rabies but many couldn’t recall having gotten sick.

World Health Organization officials have suggested hiring Ebola survivors to help treat the sick. In Sierra Leone, the United Nations is training survivors to help sick children, who often suffer through illness with no one to touch or comfort them.

“A key challenge that parents, care workers and many of us working on the Ebola response are facing is how to care for children who have been affected or infected with Ebola without putting their caregivers at risk,” Roeland Monasch, UNICEF Representative in Sierra Leone, said in a statement.

“One creative way to address this gap is to work with Ebola survivors who can provide these children with the love, care and attention they so badly need.”

Ebola's taken a serious toll on health care workers, infected more than 400 and killing more than 200 of them. British nurse Will Pooley has said he's heading back to West Africa to help after he survived Ebola, and has noted that he is now almost certainly immune to the virus.

“Ultimately, knowing whether a large segment of the population in the afflicted regions are immune to Ebola could save lives,” said Steve Bellan, who worked with Meyers on the letter published in the Lancet medical Journal. “If we can reliably identify who they are, they could become people who help with disease-control tasks, and that would prevent exposing others who aren't immune."

Or their blood could be used to treat the ill, if that approach turns out to in fact help patients survive.