ATLANTA — An ambulance transporting the fourth American aid worker sickened with the Ebola virus has arrived at Emory University Hospital for treatment.

The patient arrived about 10:20 a.m. Tuesday after a medical plane transported him to the Atlanta metro area. The patient walked to the hospital from the ambulance.

Two other U.S. aid workers have been treated at Emory. Compared to those earlier arrivals, the scene at the hospital campus was calm Tuesday.

Emory's previous two patients recovered and have been released. Another patient, a doctor, is being treated in Nebraska.

Hospital officials released no details on the latest patient, but the World Health Organization says a doctor who has been working in an Ebola treatment center in Sierra Leone tested positive for the disease. It said the doctor was being evacuated.