A U.S. health care worker, who was possibly exposed to the Ebola virus in Sierra Leone, is scheduled to arrive for observation Sunday at a hospital in the U.S. western state of Nebraska.

The medical director of the Biocontainment Unit at the University of Nebraska Medical Center in Omaha said the unidentified patient "is not ill and is not contagious."

The patient will be monitored for 21 days to see if the Ebola disease develops.

On Saturday, a London hospital said the health of a Scottish nurse being treated for Ebola had deteriorated rapidly in recent days and she is in critical condition.

Pauline Cafferkey had been volunteering in the anti-Ebola effort in Sierra Leone last month before she fell ill.

The World Health Organization says more than 20,000 people are infected with the Ebola virus. More than 7,900 people have died from the disease.

Almost all the Ebola cases have been in the three hardest hit countries - Sierra Leone, Liberia and Guinea.

The other nations where at least one Ebola case has been diagnosed include Nigeria, Mali, Senegal, Spain, the U.S. and Britain.

Some information for this report came from AFP and Reuters.