While more than 1,500 people have died from Ebola virus disease — about 53 percent of those infected in the historic outbreak spreading in West Africa — two Americans who received an experimental drug and were brought back to the U.S. for treatment survived and have returned to their families.

What was different with them? What went into caring for them? Why have others still died despite receiving the same experimental drug? And what did doctors learn that could be useful moving forward?

Scientific American sat down with Dr. Bruce Ribner, the medical director of Emory University Hospital’s Infectious Disease Unit where Dr. Kent Brantly and Nancy Writebol were treated in Atlanta, to get the insider scoop and learn what takeaways the hospital might send to areas struggling to treat and contain the outbreak.

ATLANTA, GA - AUGUST 21:  Dr. Kent Brantly, an Ebola patient at Emory Hospital, talks at a press conference about his release from the hospital on August 21, 2014 in Atlanta, Georgia.  Dr. Brantly and another patient, Nancy Writebol, were released from Emory Hospital after receiving treatment for Ebola that they both contracted while working as medical missionaries in Liberia.  (Jessica McGowan/Getty Images)

Dr. Ribner spoke at a press conference last week when Dr. Kent Brantly was being discharged from the hospital being cured of the Ebola virus disease.  (Jessica McGowan/Getty Images)

The number one difference between the treatment in America and that being offered in communities bogged down with Ebola patients is that Emory only had to treat two people. In addition, it had better infrastructure and testing capabilities available to it. Ribner said they had five physicians and 21 nurses dedicated to helping these two patients, in addition to the support of hundreds of others.

“The developed countries of the world will have to do our part to assist our colleagues with less developed infrastructure to care for sick people,” Ribner told Scientific American. “I think one of the messages that is going out from many sources is we really have to help countries such as the ones involved in this outbreak to develop their medical infrastructure. Hopefully in five years they will have this infrastructure.”

While in Emory’s care, Ribner said the team found that the two patients experienced electrolyte abnormalities due to the fluid loss they suffered. Some facilities don’t have the ability to test for electrolyte levels so, he said, they’ll be spreading awareness about the low levels of sodium and potassium come as a result of symptoms caused by the virus. They also noted liver damage leading to a lack of proteins, which Ribner said could be replaced in patients early on.

Ebola victim Dr. Kent Brantley, left,  embraces Dr. Bruce Ribner medical director of Emory’s Infectious Disease Unit, after being released from Emory University Hospital, Thursday, Aug. 21, 2014, in Atlanta. (AP/John Bazemore)

Ebola victim Dr. Kent Brantley, left, embraces Dr. Bruce Ribner medical director of Emory’s Infectious Disease Unit, after being released from Emory University Hospital, Thursday, Aug. 21, 2014, in Atlanta. (AP/John Bazemore)

As for the other people who have received the experimental ZMapp drug, which was first given to Brantly and Writebol before they were transported back to the U.S., some have still died. Ribner pointed out that’s because they don’t really know yet if the drug actually had an effect for anyone who took it.

“Experimental drugs are experimental drugs because we don’t know if they will work. That is true both with the preparations patients received in Liberia and other preparations that are being considered for treating patients with this infectious disease. We are a long way from being able to say that someone that received one of these agents benefited, it had no impact or it may be that their outcome may be impeded,” Ribner said, according to Scientific American. “Until we have good studies looking at outcomes of patients who received these medications, compared to patients who didn’t receive them, we should be very cautious.”

Ribner added that he believes the focus still needs to remain on the importance of care and treatment, “rather than receiving any magic vaccine or product that may or may not improve survival.”

A vaccine for Ebola is about to begin testing next week, the National Institutes of Health said Thursday.

Ribner told Scientific American that the hospital is continuing to monitor both Brantly and Writebol as outpatients so that the medical community can try to gain a better understanding about immunity to the Ebola virus. As of right now, he said that they likely would have developed “a very robust immunity” to at least this strain of the Ebola virus, which has five different strains.

The World Health Organization said Thursday the Ebola outbreak in West Africa eventually could exceed 20,000 cases, more than six times as many as doctors know about now. A new plan to stop Ebola by the U.N. health agency also assumes that in many hard-hit areas, the actual number of cases may be two to four times higher than is currently reported.

Bruce Aylward, WHO Assistant Director-General, speaks to the media during a press conference about the WHO briefing on the Ebola roadmap. It outlines all actions that need to be taken by affected countries and partners to bring an end to the largest and most complex recorded Ebola outbreak in history, at the European headquarters of the United Nations in Geneva, Switzerland, Thursday, August 28, 2014. (AP/Keystone/Martial Trezzini)

Bruce Aylward, WHO Assistant Director-General, speaks to the media during a press conference about the WHO briefing on the Ebola roadmap. It outlines all actions that need to be taken by affected countries and partners to bring an end to the largest and most complex recorded Ebola outbreak in history, at the European headquarters of the United Nations in Geneva, Switzerland, Thursday, August 28, 2014. (AP/Keystone/Martial Trezzini)

As of Thursday, the U.N. agency reported that 1,552 people have died from the killer virus from among the 3,069 cases reported so far in Liberia, Sierra Leone, Guinea and Nigeria. At least 40 percent of the cases have been in just the last three weeks, WHO said, adding that “the outbreak continues to accelerate.” 

The Associated Press contributed to this report.