DENVER (AP) — The president of Frontier Airlines says a nurse who was on flights between Dallas and Cleveland and who later tested positive for Ebola may have been at a more advanced stage of the illness than previously thought.

Barry Biffle emailed employees Friday about the findings by the Centers for Disease Control and Prevention. He said the CDC has assured the Denver-based airline that crewmembers on the flights are at a very low risk of exposure.

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The airline put the pilots and flight attendants on leave for 21 days, which health experts consider the outer limit of how long it would take someone exposed to Ebola to become sick.

Biffle says passengers on Amber Joy Vinson’s flight from Dallas to Cleveland on Oct. 10 and her return flight to Dallas on Monday have been notified.

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