March 15, 2015 5:20 PM EDT

Four Americans who recently returned from Sierra Leone are being monitored for the Ebola virus at the University of Nebraska Medical Center in Omaha, the hospital announced Saturday.

The patients — who may have been exposed to the disease but currently show no symptoms — will be monitored for 21 days, the maximum Ebola incubation period, Reuters reported.

An American health care worker who tested positive for Ebola was transported to the National Institutes of Health in Maryland last week and was said to be in serious condition.

There have been more than 10,000 Ebola deaths during the current outbreak, primarily in Liberia, Guinea and Sierra Leone.

 

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Write to Justin Worland at justin.worland@time.com.

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