( CNN ) -- There 's no cure for Ebola . So why have some patients walked away healthy while others in the West died ? Dr. Kent Brantly , Nancy Writebol and Dr. Rick Sacra all contracted the disease while working in Liberia -- and all survived . Spanish nurse 's aide Teresa Romero Ramos got the virus while tending to stricken patients . She too lived . But like the patients above , Thomas Eric Duncan and Spanish priest Miguel Pajares also received treatment in the West . Yet they died . While there might not be a single , conclusive answer , a series of factors may contribute to survival . Early , high-quality treatment . This may be the most critical factor in beating Ebola . The survivors in the United States all have one thing in common -- they were rushed to two of the country 's four hospitals that have been preparing for years to treat a highly infectious disease such as Ebola . Brantly and Writebol were successfully treated at Emory University Hospital in Atlanta ; Sacra was released from the Nebraska Medical Center in Omaha . Duncan did n't go to one of those four hospitals . He went to Texas Health Presbyterian Hospital in Dallas with a fever and told them he 'd recently returned from Liberia . Yet the hospital initially sent him home with antibiotics . Then , after he returned to the hospital much more ill , two nurses became sick with the virus . They have since been moved to more specialized facilities such as Emory and the National Institutes of Health in Maryland . But that does n't mean patients are doomed just because they go to a different hospital . `` Keep in mind that this is still a very deadly disease , '' said Dr. Sanjay Gupta , CNN 's chief medical correspondent . `` In West Africa , the mortality rates are above 60 % . I think it is better in the United States . But they 're not going to be zero , I think no matter where somebody is . '' Spanish nurse 's aide found to be Ebola-free in third test . Quick rehydration . After finding a hospital capable of treating Ebola , those who survive are usually rehydrated quickly . `` The most important care of patients with Ebola is to manage their fluids and electrolytes , to make sure that they do n't get dehydrated , '' said Dr. Tom Frieden , director of the Centers for Disease Control and Prevention . `` And that requires some meticulous attention to detail and aggressive rehydration in many cases . '' And if an infected patient getting proper care normally has a strong immune system , the chance of surviving goes up . But what may seem basic in the United States can be difficult to come by in West Africa , where Ebola has already killed more than 4,500 people -- largely because access to health care is limited . Yet Nigeria has successfully eradicated Ebola from the country , the World Health Organization said Monday . Unlike in Guinea , Liberia and Sierra Leone -- the combined epicenter of the oubreak -- all identified contacts in Nigeria were physically monitored every day for 21 days , WHO said . Plasma transfusions . Three Ebola patients -- Sacra , NBC cameraman Ashoka Mukpo and Texas nurse Nina Pham -- all received plasma donations from Brantly . And all three have survived . The theory is that Brantly 's plasma contains the antibodies necessary to fight the virus . `` It 's very fortunate that the three patients I 've been able to donate to , they and I share the same blood type , '' Brantly told CNN 's Anderson Cooper . `` I 'll keep doing it as much as it 's needed , as much as I can . '' There was some controversy about why Brantly did n't give plasma to Duncan . But the problem came down to blood type , Texas Health Presbyterian Hospital said . WHO to review Ebola response amid criticism of its efforts . Experimental drugs . Medication that has n't gone through clinical trials can be risky . But with a mortality rate of 50 % in the current Ebola outbreak , a WHO panel said it is ethical to offer drugs to fight the virus -- even if their effectiveness or adverse effects are unknown . The experimental drugs at the center of this Ebola outbreak are ZMapp , Favipiravir , Brincidofovir and TKM-Ebola . Brantly and Writebol both took ZMapp , and both survived . But Pajares also took ZMapp , and he died . The director of Emory 's Infectious Disease Unit cautioned against viewing ZMapp as a surefire cure . `` They are the very first individuals to have ever received this agent , '' Dr. Bruce Ribner said . `` There is no prior experience with it , and frankly , we do not know whether it helped them , whether it made no difference , or even , theoretically , if it delayed their recovery . '' Spanish nurse 's aide Teresa Romero Ramos took the anti-viral drug Favipiravir and also received antibodies from a survivor in West Africa . She is now free of the virus . In addition to getting a blood transfusion , Sacra received an experimental drug called TKM-Ebola , which the Food and Drug Administration recently approved for wider use . Duncan took a different untested drug , Brincidofovir . But he did n't receive it until six days after being admitted to the Texas hospital . Had he taken it earlier , the outcome might have been different . CNN 's Al Goodman , Elizabeth Cohen , Faith Karimi and Catherine E. Shoichet contributed to this report .
Three patients in the U.S. have received plasma from another survivor . At least four in West survived after taking experimental drugs ; two died . Early treatment at specialized facilities for Ebola has netted positive results . But mortality rate wo n't be zero `` no matter where somebody is , '' Dr. Sanjay Gupta says .