How does an American nurse contract Ebola?

Here's what's scary about the Dallas health-care worker infected with Ebola: she knew she was treating an Ebola patient. That's not supposed to happen. We've been told that Ebola can be stopped using modern medical protocols. An American health-care worker who is part of a team that knows it's treating an Ebola patient is supposed to be able to protect herself. So what happened?
 The simple answer is that the Ebola treatment protocols are complicated.