REUTERS/Umaru Fofana The names of trainee health workers are seen written on their protective suits at a World Health Organization (WHO) training session in Freetown, Sierra Leone September 30, 2014.

As news broke early Sunday morning that a second person in Dallas had contracted Ebola, a healthcare worker who had contact with the first patient, people immediately wondered: How?

We don't yet know exactly. The Centers for Disease Control and Prevention is investigating, and a breach in protocol is suspected, CDC director Tom Frieden said at a press conference on Sunday.

The infected worker "has not been able to identify a specific breach," Frieden added. "It is very concerning." He emphasized that infection-control "protocols work," but that this case makes it clear that "enhanced training" on how to implement those protocols is needed.

Removing contaminated protective equipment has been identified as one "major potential risk," Frieden said, also mentioning high-risk intensive care procedures like intubation and dialysis, which had not been performed on an Ebola patient before the treatment of the first Dallas patient. That's part of "what the [CDC] investigation will look at, but we don't yet know what it will find."

Earlier cases of infected healthcare workers may offer some additional clues about how this could have happened.

It's an overly "simplistic explanation to say that personal protective equipment does not work — that's not the right conclusion," Dr. Daniel Bausch, a professor in the department of tropical medicine at Tulane, told Business Insider in reference to other cases among healthcare workers. "I can put 12 pairs of gloves on somebody, but if they touch something contaminated and then touch their eye," all those precautions are of little help.

We have repeatedly been told that our healthcare system is prepared to contain Ebola, and a widespread outbreak here remains highly unlikely. But secondary cases, especially in people who have close contact with patients, are expected.

"We knew a second case could be a reality, and we've been preparing for this possibility," said Dr. David Lakey, commissioner of the Texas Department of State Health Services, in a statement.

People wearing full protective gear are often at the most risk when they are removing it, which is a very involved process. Teresa Romero, a Spanish nursing assistant who contracted Ebola after cleaning the room of an Ebola patient in Madrid, told the press she thinks she may have touched her face with a contaminated glove while removing her protective suit.

"Personal protective equipment needs to be carefully removed without contaminating oneself, one’s eyes, one’s mucous membranes or clothing," Dr. David Kuhar, a medical officer in the CDC’s Division of Healthcare Quality Promotion, said on a call with clinicians in August. "I f personnel are using personal protective equipment that they’re not familiar with, they need to be trained and ready to remove it safely."

The nurses' union in Spain alleged that some workers dealing with Ebola patients there received nothing more than a 15- to 20-minute talk on how to put on the protective suits. The extent of the training that was received by workers at the hospital in Dallas is not known.

"Doing this right 100% of the time requires a very intensive training and followup process," Frieden said on Sunday.

In West Africa, where Ebola has infected more than 8,000 people and killed more than 4,000, at least 401 healthcare workers have been infected, and 232 have died. While some were infected because they didn't realize at first that a patient had Ebola, or because they lacked appropriate protection, most are unsure about exactly how they contracted the virus.

" Every day I'm still thinking, When was I contaminated?" Senga Omeonga, a doctor at a hospital in Monrovia told Science magazine.

Nancy Writebol, an American healthcare worker who contracted Ebola in Liberia (and has since recovered), thinks a coworker who thought he had typhoid, but actually had Ebola, may have infected her. Kent Brantly, an American doctor who also survived Ebola, thinks he was exposed while working in the emergency room, not while wearing full protection in the Ebola treatment unit.

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