Story highlights The WHO describes the Ebola outbreak as a "dire emergency"

Patient is a U.S. citizen who got 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

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.

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.