Dr. Rick Sacra, the third American to contract Ebola, landed in Nebraska last week and will be moved to the Nebraska Medical Center in Omaha for treatment.

Experts insist there is no risk to the public, NBC News reports, but a new study in PLOS Currents finds that Ebola could soon make its way inside U.S. borders on its own.

The study looked at global flight patterns and passenger screening and found that the chance of at least one case arriving in the country by Sept. 22 was as high as 18 percent, NPR reports.

"What is happening in West Africa is going to get here. We can't escape that at this point," the study's lead author says, adding it would likely occur in "small clusters of cases, between one and three." The study also points to a 25 percent to 28 percent chance of the virus reaching the United Kingdom and a 50 percent chance of it spreading to Ghana before the month is over.

If the virus isn't contained, the likelihood of its spread will "increase consistently," the study notes. On a more optimistic note, Sacra's wife says, "Rick is clearly sick" but "was in good spirits and he walked onto the plane" that took him to Nebraska.

"We are really encouraged by that news." (Meanwhile, Sierra Leone is going on lockdown to fight Ebola.)

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