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How some patients manage to survive Ebola

Liberian health workers are seen at the Medecins Sans Frontieres (Doctors Without Borders) Ebola treatment center in Monrovia, Liberia, on October 18, 2014.

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The Ebola virus has a horrific death rate -- it is fatal in about 70 percent of patients in the current outbreak in West Africa, according to estimates from the World Health Organization.

While Thomas Eric Duncan, the first patient diagnosed in the U.S., died, there have been success stories, too: A nursing assistant infected in Spain has recovered, as have four American aid workers infected in West Africa. Even there, not everyone who falls ill with Ebola dies.

It's not clear why Ebola runs a different course in different people. But how rapidly symptoms appear depends partly on how much virus a patient was initially exposed to, said Dr. Joseph McCormick of the University of Texas School of Public Health. Formerly with the Centers for Disease Control and Prevention, McCormick worked on the first known Ebola outbreak in 1976 and numerous other outbreaks of Ebola and related hemorrhagic viruses.

"It's obviously a scary disease if you have it," Andrew Noymer, a public health professor who studies infectious diseases at University of California, Irvine, told CBS News. "But it's just not highly transmittable. I can't say that enough."

The World Health Organization has made clear that there's far more virus in blood, vomit and feces than in other bodily fluids.

Ebola spreads by contact with bodily fluids, such as through a break in the skin or when someone with contamination on their hands hands touches the eyes or nose. Once inside the body, Ebola establishes a foothold by targeting the immune system's first line of defense, essentially disabling its alarms. The virus rapidly reproduces, infecting multiple kinds of cells before the immune system recognizes the threat and starts to fight back.

Only after enough virus is produced do symptoms appear, starting with fever, muscle pain, headache and sore throat. Those symptoms typically set in after an incubation period of 8 to 10 days, although it can be as long as 21 days after exposure. And only then is someone contagious.

There is no specific treatment for Ebola but specialists say basic supportive care -- providing intravenous fluids and nutrients, and maintaining blood pressure -- is crucial to give the body time to fight off the virus. Survival also can depend on how rapidly someone gets care.

Profuse vomiting and diarrhea can cause dehydration. Worse, in the most severe cases, patients' blood vessels start to leak, causing blood pressure to drop to dangerous levels and fluid to build up in the lungs.

"The key issue is balance between keeping their blood pressure up by giving them fluids, and not pushing them into pulmonary edema where they're literally going to drown," McCormick said.

Death usually is due to shock and organ failure.

"We depend of the body's defense to control the virus," explained Dr. Bruce Ribner, who runs the infectious disease unit at Emory University Hospital in Atlanta and led the team that treated two American aid workers who contracted the virus in Liberia over the summer. "We just have to keep the patient alive long enough in order for the body to control this infection."

What about experimental treatments? Doctors at Emory and Nebraska Medical Center, which successfully treated another aid worker and now is treating a video journalist infected in West Africa, say there's no way to know if those treatment really helped.

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Dr. Kent Brantly during a press conference announcing his release from Emory University Hospital on August 21, 2014 in Atlanta.
Jessica McGowan, Getty Images

Treatment options have included plasma transfusion, donated by Ebola survivors who have antibodies in their blood able to fight Ebola. One of the aid workers who survived, Dr. Kent Brantly, has donated blood for transfusion to several subsequent patients, but was unable to help Thomas Eric Duncan because their blood types weren't a match.

Several of the aid workers also received an experimental drug called ZMapp that has not yet been tested in humans. However, a Liberian doctor who also received it still died. The very limited supply of ZMapp ran out in August and it will take another month or two until more is available.

Survival may also may be affected by factors beyond anyone's control: McCormick's research suggests it partly depends on how the immune system reacts early on -- whether too many white blood cells die before they can fight the virus. Other research has linked genetic immune factors to increased survival.