On the surface, the nurse seemed to have taken all the precautions needed to protect herself from Ebola.
She wore a mask, gown, shield and gloves. Her patient was isolated in an American hospital.
And yet the woman still contracted Ebola, marking the first known transmission ever in the United States.
The Centers for Disease Control and Prevention say there was a breach in protocol, but haven't elaborated on what it means. Instead, it said the protocols laid out for American hospitals work.
So what happened? How could a nurse at an American hospital contract the virus? And how troubling is it?
How it happened
CDC officials have spoken to the infected nurse, and she wasn't able to point to a specific breach.
"At some point, there was a breach in protocol, and that breach in protocol resulted in this infection," the director of the Centers for Disease Control and Prevention, Dr. Thomas Frieden, said at a news conference Sunday.
The nurse, who works at Texas Health Presbyterian Hospital in Dallas, had been treating Thomas Eric Duncan - the first patient ever diagnosed with Ebola in the United States. Duncan died Wednesday.
The CDC is looking at several possibilities as to how that breach may have happened.
The agency says it will be looking at two areas in particular:
a) whether the infection occurred during kidney dialysis or respiratory intubation.
Duncan underwent both procedures "as a desperate measure to try to save his life," Frieden said. "Both of those procedures may spread contaminated materials and are considered high-risk procedures."
b) whether the infection occurred during the removal of protective equipment.
"When you have potentially soiled or contaminated gloves or masks or other things, to remove those without any risk of any contaminated material ... touching you and being then on your clothes or face or skin ... is not easy to do right."
Or the problem could have been something else entirely.
CNN Senior Medical Correspondent Elizabeth Cohen said precautions among health care workers can actually go overboard sometimes.
"For example, they are supposed to double glove in some situations. Well, triple gloving is a violation of protocol and actually could make things worse, instead of making things better, because then you need to take off three pairs of gloves ... gloves with infectious stuff on them."
See more on this developing story on CNN.com