Atlanta ( CNN ) -- Another patient with Ebola is heading to the isolation unit at Emory University Hospital in Atlanta . An air ambulance will fly the patient from West Africa and is expected to arrive on Tuesday morning , Emory Healthcare said in a statement . University officials said Monday that they could not reveal additional details because of confidentiality restrictions . The State Department described the patient as a U.S. citizen who contracted the disease in Sierra Leone . Earlier Monday , the World Health Organization said a doctor working in an Ebola treatment center in Sierra Leone had tested positive . WHO would not reveal the nationality of the doctor nor to where he or she was being evacuated . Last month , two Americans infected with Ebola were flown to Atlanta for treatment at Emory . They were given an experimental drug to fight the deadly disease and were later released from the hospital . Another American doctor infected with Ebola arrived in Nebraska for treatment last week . Officials have warned that the deadliest Ebola outbreak in history is hitting West Africa . WHO said Monday the rapid spreading of the virus in Liberia shows no sign of slowing . `` The number of new cases is increasing exponentially , '' WHO said , calling the situation a `` dire emergency with ... unprecedented dimensions of human suffering . '' It 's a crisis unlike any other during previous Ebola outbreaks , according to WHO . Taxis packed with families who fear they 've contracted the deadly virus crisscross the Liberian capital , searching for a place where they can be treated , the WHO said . But there are no free beds . `` As soon as a new Ebola treatment facility is opened , it immediately fills to overflowing with patients , '' the U.N. group said . The Ebola virus causes viral hemorrhagic fever , which according to the U.S. Centers for Disease Control and Prevention refers to virus types that affect multiple organ systems in the body and are often accompanied by bleeding . Humans contract Ebola through contact with the bodily fluids of infected animals or the bodily fluids of infected humans . No vaccine or medicine has been proved to cure the disease , but the first human trial of an experimental Ebola vaccine began last week . More than 3,600 cases of Ebola have been reported since the first documented case in December , WHO has said . Of those cases , there have been more than 1,800 fatalities , the agency has said . More than 40 % of the cases have been diagnosed in the past three weeks , the agency said . CNN 's Miriam Falco and Debra Goldschmidt reported from Atlanta . CNN 's Jamie Crawford reported from Washington . CNN 's Susannah Cullinane , Madison Park , Carol Jordan , Catherine E. Shoichet and Chelsea J. Carter contributed to this report .
NEW : The WHO describes the Ebola outbreak as a ` ` dire situation '' Patient is a US citizen who contracted Ebola in Sierra Leone , State Department says . Emory says an air ambulance will transport an Ebola patient to Atlanta . The patient will be brought to a hospital isolation unit , Emory says .