Amber Vinson’s blood tested negative for Ebola virus just nine days after she was first diagnosed. Her fellow nurse, Nina Pham, is now in good condition.

The two nurses infected when they treated the first person diagnosed with Ebola in the United States, Thomas Eric Duncan, have seemingly fared better than most patients with the disease. Is that thanks to early treatment, did the two nurses just get a smaller dose of the virus to start with, or are other factors at work?

Doctors familiar with Ebola say it’s almost impossible to know. Only seven people have ever been treated for Ebola in the United States. Six recovered and only Duncan died. Even with such small numbers it’s a stark contrast to the epidemic in Liberia, Sierra Leone and Guinea, where the World Health Organization says 70 percent of patients are dying.

“It is rare that recovery happens this fast,” said Thomas Geisbert, an expert on infectious diseases and Ebola treatment at the University of Texas Medical Branch. “It could be related to a number of things including the fact that these patients were diagnosed in the U.S. and treatment was presumably initiated quickly,” he told NBC News.

But even in Africa, with late and minimal treatment, some people do pull through, says Kathryn Jacobsen, a George Mason University epidemiologist who has seen it first hand in Bo, in Sierra Leone.

“While most people who contract Ebola virus become severely ill, some people have relatively mild infections,” Jacobsen told NBC News. “It is not possible in this case to know whether the infected nurse had a speedy recovery because of early medical interventions or because she happened to be someone with an immune system that allowed her to fight the infection relatively quickly.”

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But groups such as Medecins Sans Frontieres (MSF or Doctors Without Borders) say they have found that when people are diagnosed fast and get treatment quickly, even with the limited facilities available in West Africa the death rate can be lowered measurably.

“Early medical intervention is likely to be associated with a better outcome. One of the continuing challenges in Guinea, Liberia, and Sierra Leone is that there are too few hospital beds available for Ebola patients,” Jacobsen said. “Home-based care is sometimes the only option available to people infected with Ebola in West Africa, and that generally means having no access to IV fluids, oxygen, or antibiotics for secondary bacterial infections, which are all therapies that improve Ebola survival rates.”

What beds are available go to the sickest patients, meaning early treatment just doesn’t happen.

In Africa, the treatment’s almost completely supportive, and patients often cannot even be given saline to replace lost fluids. Doctors who treated Dr. Kent Brantly at Emory University Hospital say they found he had almost cholera-like symptoms, with severe diarrhea causing loss of important compounds called electrolytes, which are important for heart, nerve and other organ function.

They felt that measuring and precisely replacing these electrolytes made a big difference. Brantly also got what’s called convalescent serum — blood taken for an Ebola survivor — as well as an experimental drug called ZMapp.

Yet Brantly took far longer to recover than Vinson. He was diagnosed July 26 and not released until Aug. 21. One big difference — his early days of treatment took place in Liberia before he was stable enough to be flown to the U.S.

Fellow medical missionary Dr. Rick Sacra got a dose of Brantly’s antibody-laden blood plus an experimental drug, yet spent 23 days in the Nebraska Medical Center. Freelance journalist Ashoka Mukpo, released Wednesday, also got some of Brantly’s blood and also an experimental drug called Brincidofovir and it was 20 hard days for him. He says he’s still weak.

But for all of them, unlike Vinson and Pham, their first few days of care took place in Africa. And Duncan, the patient who died, was sick for at least four days without treatment before he was admitted to Texas Health Presbyterian hospital in Dallas. Could those first few days be the key?

Dr. Nahid Bhadelia, an infectious disease specialist at Boston University who’s treated Ebola patients in Sierra Leone, says there’s a not a lot of information because the response in African outbreaks has been a frantic effort to save lives rather than an academic exercise in gathering data.

How quickly the body fights off any virus depends on several factors, such as how the virus was transmitted.

“For example, we have seen that those who have a needlestick injury with Ebola do worse than those with just exposure via mucous membrane,” she said. The patient’s baseline health is a big factor, also.

“With Ebola, it is possible that early care can provide integral support to the body while it mounts its immune response, hence allowing it to clear the virus quickly,” Bhadelia said. “We know for a fact that early intervention can help decrease mortality in most cases. In the case of the two nurses, I think it’s probably a combination of these factors.”

There’s also some study that suggests some people can be exposed to the virus and never even get sick.

Geisbert says it’s an argument for watching contacts of cases closely, so they can get immediate treatment, and for building labs and clinics in West Africa.

“I am a strong believer that early intervention can not only decrease mortality in the person affected but it may even help us control the spread as sicker patients have more virus, which can be more readily spread to caretakers, etc.,” he said.