The nurse might have had contact with the priest before he was isolated, or not followed isolation rules, or might simply have been very unlucky BUT she has been tested early and won't have had a chance to start displaying symptoms and to infect a number of people. A lot of the problem in Africa is that people become ill in their homes and infect their families before they can be isolated, or are transported to hospital by taxi which then isn't disinfected before others use it, or are removed from clinics and taken home because their families are frightened and want to care for them in the traditional way. There's no early testing, no careful isolation, and far less public awareness of how to safely care for possible sufferers and get them safely to a treatment centre.
Did you know there's a growing community of ebola survivors in Africa who are helping doctors analyse what it is about them that allowed them to survive the disease? And that's just one of the avenues for treating and preventing this strain of ebola. There is hope :)