(CNN) -- How did this happen?
That is what many people were asking Wednesday, after news broke that a health care worker had flown commercially at a time she was supposedly monitoring herself for a possible Ebola infection.
Amber Vinson, 29, is the third person to be diagnosed with Ebola in the United States -- after Thomas Eric Duncan, the Liberian national who fell ill after coming to Texas, and Nina Pham, who like Vinson had cared for Duncan at Dallas' Texas Health Presbyterian Hospital.
As with Pham, many questions surround how Vinson contracted Ebola despite efforts to prevent health care workers like them from getting the disease. Now they are compounded with others about safeguards for those who had contact with Duncan, who succumbed to the disease on October 8, to prevent the disease from spreading further.
One of authorities' biggest fears is that Ebola could be transmitted to the general public in a confined, crowded place. Someplace like, perhaps, a busy airport or an airplane -- like that of Frontier Flight 1143, which Vinson flew Monday with a slightly elevated temperature from Cleveland to Dallas.
Should Vinson have flown?
No, according to the head of the Centers for Disease Control and Prevention.
Dr. Thomas Frieden said -- given what happened with Duncan and a nurse who got Ebola from him -- the health care care worker should not have traveled on a commercial airline.
"The CDC guidance in this setting outlines the need for what is called controlled movement," Frieden said. "That can include a charter plane, a car, but it does not include public transport."
Is it possible she didn't know that?
This is a big question, because it's harder to fault Vinson if no one instructed her not to fly commercially.
Another infectious disease expert -- Dr. Art Reingold, head of epidemiology at the University of California-Berkeley School of Public Health -- says he hadn't heard "any firm recommendations" on traveling for those, like Vinson, who were supposed to be monitoring themselves.
"If I had travel plans, I would probably put them on hold," Reingold said. "But I haven't heard anyone say that should be a policy."
Did Vinson talk to authorities before getting on the plane?
Yes, according to a federal official.
Vinson called the CDC on Monday to report she had a temperature of 99.5 Fahrenheit and inform the agency she was about to board a plane back to Dallas, the official said.
No one at the CDC told her she couldn't or shouldn't fly, according to the official.
Was Vinson contagious when she flew?
A person can't pass on Ebola the moment they contract it. Rather, someone is only contagious when they are showing symptoms -- which, according to the CDC, include a severe headache, muscle pain, diarrhea, vomiting, abdominal pain and a fever greater than 101.5 degrees Fahrenheit (38.6 degrees Celsius). And the National Institutes of Health notes that a temperature over 100.4 degrees usually means a person has a fever caused by an infection or illness.
That latter measure -- whether a person's temperature is 100.4 degrees or higher -- is the first item on a CDC checklist for those being evaluated for Ebola. The CDC is considering lowering that threshold for the virus, according to an official familiar with the situation.
What's not known is how that varies from her normal body temperature (a baseline that can differ depending on the person, time of day or other factors), how it might have spiked during the flight or how she was feeling beyond her temperature.
And the fact of the matter is Vinson wasn't just any traveler. She, like fellow Ebola sufferer and Dallas health care worker Nina Pham, had "extensive contact" with Duncan between September 28 and 30 when he had "extensive production of body fluids" such as vomit and diarrhea, Frieden said.
In other words, Vinson had special reason to be careful about her interactions with others.
Why aren't all travelers temperatures checked in Cleveland?
Because that's not the U.S. policy -- at least right now.
Passengers' temperatures are checked at five American airports: New York's Kennedy, Newark's Liberty, Atlanta's Hartsfield-Jackson, Chicago's O'Hare and Washington's Dulles. Cleveland's Hopkins International Airport is not on the list.
Even then, those temperature checks are only for those who have come from West Africa to the United States. Even if Vinson had flown from, say, O'Hare to Dallas, she wouldn't have been checked because she hadn't recently flown internationally.
Could this policy change? Sure. But Dr. Joseph McCormick points out that testing temperatures wouldn't catch those, like Duncan, who got exposed days before and didn't show any symptoms while traveling. It would end up getting many more people who have fevers for other reasons, especially once flu season kicks into high gear.
"I think that's more than enough," McCormick, dean of the University of Texas School of Public Health's Brownsville campus, said of the current checks on those coming from West Africa. "I'm not persuaded (testing all travelers' temperature) is going to be really doing anything."
Are those on that Frontier Flight 1143 in danger?
There were 132 people who flew, with Vinson, on a Frontier flight from Cleveland to Dallas earlier this week. And each one of them will be interviewed by authorities and could be monitored. But that doesn't mean they are likely to show symptoms of Ebola.
The virus doesn't spread, through the air, like the flu or cold. It is transmitted through bodily fluids such as urine, saliva, sweat or feces, none of which you'll likely exchange with every fellow passenger on your flight.
Still, a lot depends on where you were sitting and what symptoms Vinson had during the flight. If you rubbed elbows with her, mistakenly shared a drink or something else, that might be cause for heightened for concern.
"If I had been on that plane, I personally would not have been worried about getting Ebola," Reingold said. "Now, if that person vomited on my lap, I would be saying something quite different."
So who could have prevented this?
This would never have happened had Duncan not contracted Ebola in Liberia, then came down with symptoms from the virus in Texas. And we wouldn't be talking about Vinson's travels had she not gone to Cleveland, or tried to come back to Dallas.
But there seems to be one thing that everybody agrees on: Health authorities made mistakes themselves.
Yes, the CDC is the top federal agency when it comes to infectious diseases. But as McCormick points out, it doesn't have any legal authority over state or local health agencies. So any policies it suggest can be moot, unless embraced at a more local level.
And obviously, such policies -- like, perhaps, that those being asked to "self-monitor" for Ebola not fly commercially -- are more futile if they're not communicated at all.
McCormick said "the buck stops" with the hospital, which has the primary responsibility to train and prepare its staff. He finds Texas Health Presbyterian Hospital officials' response to the latest case -- and the fact a CDC official wasn't there alongside them -- to be "disappointing."
"They were really pretty much trying to paper over the missteps," McCormick said. "... The evidence is just overwhelming is that this is not good."
Is anything going to change?
If you listen to and believe government authorities, then yes.
The CDC has established Ebola response teams to send to hospitals within hours of a new case and promised to give hospitals a clear understand of how best to protect their workers. Clear communication is a big component in preventing the disease's spread, much like proper equipment.
Sylvia Burwell, the U.S. Secretary of Health and Human Services, says government policy will continue to evolve every time there's a new case, a new mistake or a new breakthrough.
"Wherever possible we're learning from our experiences and doubling on our efforts to continue to improve processes and procedures," Burwell said Wednesday. "We're continuing to communicate and supporting implementation efforts on the ground."
That's important because, as the people of West Africa know all too well and the U.S. cases show, the margin for error is slim when it comes to Ebola.
As Dr. Frank Esper, an infectious disease specialist in Cleveland, says: "Ebola is a disease that is not very forgiving of mistakes."
CNN's Ryan Nobles, Miriam Falco and Elizabeth Cohen contributed to this report.