(CNN) -- 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 say there was a breach in protocol, but haven't elaborated on what it means. Instead, it said the protocols in place at American hospitals work.
So, what happened? How could a nurse at an American hospital, trained in treating an Ebola patient, 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.
It's not clear exactly how that breach happened. But the CDC is looking at several possibilities.
The agency says it will be looking at two areas in particular:
a) whether the infection occurred during kidney dialysis and respiratory intubation procedures.
Duncan underwent both "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 taking off 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."
It could be something else entirely.
CNN Senior Medical Correspondent Elizabeth Cohen said sometimes precautions among health care workers could actually go overboard.
"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."
How troubling is it
The CDC concedes it is worrisome.
"It is possible in the coming days that we will see additional cases of Ebola," Frieden said.
That's because others who provided care to Duncan could have had the same kind of breach as the infected nurse.
The public areas of the nurse's apartment complex have been decontaminated. Officials have notified her neighbors. And the city of Dallas made a reverse-911 call to residents, notifying them of the city's second Ebola case.
"I think it needs to be put into context that ... you can't make an extrapolation that this is now a danger more so to the American people," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, cautioned.
But the fear is spreading.
When a passenger on a flight from New York to Los Angeles fell ill Sunday, the biggest concern was Ebola. The plane was taken to a remote gate at Los Angeles International Airport. After a few hours, officials determined the woman did not appear to have Ebola, and other passengers were allowed to leave.
And in Louisiana, Attorney General Buddy Caldwell is seeking a temporary restraining order to block the disposal of incinerated waste from Duncan's personal items at a Louisiana landfill.
"There are too many unknowns at this point, and it is absurd to transport potentially hazardous Ebola waste across state lines." Caldwell said Sunday.
What is being done
What's needed is more training.
President Barack Obama wants federal authorities to immediately take further measures to ensure health care workers are able to follow protocols for treating Ebola patients.
Of the thousands of hospitals in the United States, only four have been training for years to deal with highly infectious diseases like Ebola: Emory University Hospital in Atlanta, The Nebraska Medical Center, the National Institutes of Health in Maryland, and the Rocky Mountain Laboratories in Montana.
"They have the management, the processes, the implementation in place that if an Ebola patient comes in, just right away they know what to do," Gavin MacGregor-Skinner, who teaches public health preparedness at Penn State University, said.
But if someone with symptoms of Ebola shows up at any other hospital, as Duncan did, the hospital might not be ready.
"It may not be that every single hospital is in fact prepared for this," said David Sanders, associate professor of biology at Purdue University.
"We may have to think about regional centers that are best prepared to deal with Ebola patients.
Another step: screening.
On Saturday, passengers arriving from the three nations hardest hit by Ebola started getting special screening, including having their temperature taken, at New York's JFK airport.
Washington's Dulles, Newark, Chicago's O'Hare and Atlanta international airports will begin screening Thursday.
Where it's falling short
The country's largest nursing organization, National Nurses United, surveyed 2,000 members. Seventy six percent said their hospital hasn't communicated any policy about how to admit Ebola-infected patients. Eighty five percent said their hospital hasn't provided education on Ebola where the nurses can interact and ask questions.
Nurses "are alarmed at the inadequate preparation they see at their hospitals," said the group's executive director, RoseAnn DeMoro.
"The time to act is long overdue," DeMoro said.