(NBC News) While health care workers in west Africa try to gain control of the worst outbreak of ebola in history, doctors and nurses here in the United States are on alert just in case the deadly virus is found in this country.

Part of the emergency room at Carolinas Medical Center in Charlotte, North Carolina was closed briefly Wednesday when a patient who’d traveled to a country known to be battling infectious disease arrived feeling ill.

Doctors determined the patient did not have ebola.

“People should realize that the risk of having an outbreak of ebola in the United States is exceedingly unlikely and rare, in part because of the infrastructure that we have for good infection control,” says Dr. Martin Cetron of the Centers for Disease Control.

Ebola is a virus that kills 60 to 90-percent of its victims. It’s spread through direct contact with the bodily fluids of an infected person, even if that person has died.

Doctors and nurses are trained to know the symptoms: Fever, headache, diarrhea, vomiting. So are the volunteers who are coming back to the United States after serving in Africa.

The two American workers infected with ebola in Liberia, Dr. Kent Brantly and Nancy Writebol, have shown slight improvements in the past 24 hours.

Both remain in Africa in serious condition.

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