Authorities are trying to trace 132 passengers who unknowingly travelled on a plane with a nurse infected with Ebola in the US.
The second health worker who caught the virus while caring for a dying patient in Dallas, Texas, took a Frontier Airlines flight from Cleveland, Ohio, to Dallas-Fort Worth on Monday.
It was just one day before she reported developing symptoms of infection, officials said on Wednesday.
The Centers for Disease Control and Prevention (CDC) and the airline said they are attempting to contact all passengers on the flight.
The nurse had been treating Liberian Thomas Eric Duncan, who became the first person to be diagnosed with Ebola in the US after travelling to visit relatives.
Crew members on the plane said she showed no symptoms of Ebola during the flight but the next morning she developed a high temperature, and on Tuesday night tested positive for the virus.
Infected Ebola patients are not considered contagious until they have symptoms as the disease is transmitted through contact with the bodily fluids of an infected person.
The CDC is asking the passengers to contact them to be monitored.
"Health officials have interviewed the latest patient to quickly identify any contacts or potential exposures, and those people will be monitored," the department said in a statement.
News of the second nurse's diagnosis followed criticism of the hospital's initial handling of the disease.
On Tuesday, American association National Nurses United said: "The nurses strongly feel unsupported, unprepared, lied to, and deserted to handle the situation on their own."
The hospital lacked protocols to deal with an Ebola patient, offered no advance training and provided them with insufficient gear, including non-impermeable gowns, gloves with no taping around wrists and suits that left their necks exposed, it claimed.
Representatives from the Texas Health Presbyterian Hospital said it was doing everything it can to contain the virus and did not have "a systematic institutional problem".
Ebola has killed more than 4,000 people in the worst-affected countries of Guinea, Liberia and Sierra Leone, according to the World Health Organisation, which warned that there could be up to 10,000 new cases diagnosed in West Africa by December if efforts to stop it are not stepped up.
Additional reporting by agencies
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