The most in-depth look yet at the patients suffering in the current Ebola epidemic provides some important clues about how the virus kills.

It confirms that bleeding, the most horrific symptom of Ebola, is very rare. It also confirms that people may not be contagious early on in the illness, and that treatment, even basic treatment, can save lives. And it shows the incubation period is usually much shorter than the 21 days many focus on — more like six to 12 days.

A team of virus experts who have been in Sierra Leone since the epidemic spread there earlier this year documented what they found in the first 106 patients treated across the country. It’s the older patients who die the most often — 94 percent of people over age 45 died in Sierra Leone, the researchers report in the New England Journal of Medicine. More than half of those under the age of 21 lived.

“It is really the first time this is possible in an outbreak setting, since we were there,” says Robert Garry, a virologist at Tulane University who’s part of the Viral Hemorrhagic Fever Consortium. “We had people willing to take those measurements, fill out the forms, we had the laboratory set up there,” he told NBC News.

More than 70 percent of the first patients in Sierra Leone died — a percentage that has stayed consistent. On Wednesday the World Health Organization said 13,000 people had become infected in the epidemic in Sierra Leone, Liberia and Guinea, and said 70 percent are dying.

"I think you can feel a little off and there won’t be that much virus in the blood, if any."

Their real-time look at Ebola paints a clear picture of how the epidemic’s killing people. “There is a good correlation between how much virus a person has in their blood when they come in and whether they live or die,” Garry said.

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That doesn’t necessarily mean that people who come in after they’ve reached the later stages of disease, Garry said. Some people may be naturally more able to fight off the virus. “People that have fatal infections, they generally are not able to handle the virus, so the viral loads stay high,” he said.

There are also some measurements of organ damage that can help doctors predict who’s more likely to die. The virus circulates in the blood, but it attacks the liver and kidneys and perhaps other organs, too. Measurements of this organ damage — compounds such as creatinine and liver enzymes — were worse in the people who died.

"There is definitely liver damage, kidney damage and (damage to) other internal organs," Garry said. "The virus may well be replicating there."

Measurements of carbon dioxide also showed who was in the most trouble. Carbon dioxide builds up when people pant, a sign they're struggling to breathe.

The study also confirmed what doctors have been saying in the argument against quarantining people traveling back to the U.S. from West Africa. If people are not really sick — vomiting, or suffering from diarrhea — they also have very little virus in their blood. That supports the argument that people are not contagious to others when they just feel funny, or even if they have a fever.

"I think you can feel a little off and there won’t be that much virus in the blood, if any," Garry said.

Perhaps most important, the study found that even basic treatment does save lives. Most of the patients got intravenous saline to replace fluids lost to vomiting and diarrhea, oral antibiotics to fight off other infections they might have, and supportive care.

“People have the perception that if they are going to one of these treatment centers, that they are just going there to die."

“We saw the case fatality rate was definitely lower in patients that getting this treatment,” Garry said. “People have the perception that if they are going to one of these treatment centers, that they are just going there to die. That perception needs to be changed.”

The study also confirmed what the symptoms are — 89 percent of the patients had fever, 80 percent had headache, 66 percent felt weak, 60 percent were dizzy, 51 percent had diarrhea, 40 percent had abdominal pain and 34 percent had vomiting. Only one patient had bleeding.

The study was only possible because Kenema Government Hospital was already up and running to do research on a different tropical fever — Lassa fever. “You have to have someone there to take the measurement, watch the patient, and write it down in a chart,” Garry said. That wouldn’t have been possible if the U.S. government and other funders of the Viral Hemorrhagic Fever Consortium hadn’t paid for the team to be there long before Ebola broke out.