DENVER — 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, according to the president of Frontier Airlines.

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.

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 said 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. VInson, 29, reported Ebola-like symptoms on Tuesday, and later tested positive for the disease. She is currently “ill, but clinically stable,” according to the CDC.

CDC director Tom Frieden said on Wednesday that Vinson “should not have traveled on a commercial aircraft” because she had been exposed to Ebola while caring for Thomas Eric Duncan, the first person to be diagnosed with Ebola in the United States. Duncan died last week.

Nina Pham, another nurse, was also infected after treating Duncan.

Additional reporting by Mashable

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