A second American aid worker infected with the Ebola virus arrived Tuesday in Atlanta from Liberia, as Mount Sinai Hospital officials awaited results of tests on a man quarantined who had recently been to a West African country where there has been an outbreak of the deadly virus.

The hospital said that the patient, who remains in isolation, "was stable overnight and in good spirits. No other patients have presented with similar symptoms and travel history to West Africa."

He came to the emergency department early Monday morning with a high fever and gastrointestinal symptoms.

Specimens were sent to the national Centers for Disease Control and Prevention in Atlanta on Monday. The hospital said it would take 24 to 48 hours for results.

Dr. Mary Bassett, New York City's health commissioner, said that "looking at his history, we think it's highly unlikely that he has Ebola, but we're going to await the tests from the CDC."

In the meantime, aid worker Nancy Writebol, 59, arrived from Monrovia, Liberia, and was taken by ambulance to Emory University Hospital.

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Bruce Johnson, president of SIM USA, the aid group with which she was working, said Writebol is weak but shows signs of improvement.

Three days earlier, the other American aid worker diagnosed with the virus, Dr. Kent Brantly, 33, arrived at Emory.

The two patients were infected despite taking precautions as they treated Ebola patients in West Africa, where the virus has spread to four countries: Guinea, Sierra Leone, Liberia and Nigeria.

The death toll for the outbreak is 887, according to the CDC. So far Nigeria, Africa's most populous nation, has had only one death, Patrick Sawyer, 40, a Liberian-American, who flew from Monrovia to Lagos.

Eight people in Nigeria who were in direct contact with him now have symptoms of the virus and have been placed into quarantine, a Nigerian health official said Tuesday. Of the eight, only a doctor who treated Sawyer has so far tested positive. Results are pending for the others.

Except for supportive care, there is no approved treatment for Ebola. Both of the American aid workers have been treated with an experimental treatment called ZMapp, developed with U.S. military funding by a San Diego company.