The nurse has become ill within the time period of incubation for Ebola. This means that somehow she failed in keeping proper protocol when caring for the priest, or that the protocol they were using was incorrect in the first place.
http://www.bbc.com/news/world-europe-29516882
BBC is reporting they used only LEVEL 2 not LEVEL 4 contamination suits. There are photos of the nurses and they are not wearing the proper protection.
This is the “fox” in the hen house - to use your original phrase.
or she got it on the train.