The CDC emphasizes there is "little risk (from Ebola) to the U.S. general population," but with global travel in mind, it sends a health alert notice. The notice reminds health care workers to take precautions, to prepare for the "remote possibility that one of those travelers could get Ebola and return to the U.S. while sick." Health care workers should ask about a patient's travel history, know the symptoms of Ebola and know about infection control. "While it's clear there is an increased risk for working with Ebola patients, we're confident that the standard of care in the U.S. would prevent much of the transmission if a case were to show up here," said Stephan Monroe from the CDC.

The CDC also issues an alert warning travelers to avoid hospitals with Ebola patients and funerals in Liberia, Sierra Leone and Guinea.

MSF's Kamiliny Kalahne said there has never been a confirmed case of Ebola spreading to a developed country.

"This is because people generally transmit the infection when they are very sick, have a high fever and a lot of symptoms -- and in these situations, they don't travel... and even if they do get sick, once they travel to a developed country, they will be in a good hospital with good infection control, so they are very unlikely to infect others," she said.

CNN's Dr. Sanjay Gupta predicts Ebola will come to the U.S. through travelers who don't know they're infected. "It's going to happen at some point," he said.