In this photo taken Feb. 4, 2015, biologist Olivier Mbaya works with serum samples from healthy volunteer participants in a European study of an experimental Ebola vaccine, at the Vaccine Research Center at the National Institutes of Health in Bethesda, Md. It took 16 years of twists and turns. Over and over, Dr. Nancy Sullivan thought she'd finally gotten her Ebola vaccine right, only to see the next experiment fail. But it was those failures that Sullivan credits for helping her unravel enough mysteries of the immune system to finally bring an Ebola vaccine to large-scale testing in Africa. She just hopes it was in time to prove if the shots really work. (AP Photo/Cliff Owen)