A top US health official has said a mistake was "clearly" made by staff treating a man who died of Ebola in Texas, resulting in one being infected.
The female health worker infected is in an isolation ward in stable condition, awaiting confirmation of her diagnosis.
Dr Tom Frieden, head of the Centers for Disease Control and Prevention (CDC), said a full inquiry would be made into how the transmission occurred.
He said 48 other people who may also have had contact were being observed.
The health worker at Texas Health Presbyterian Hospital wore full protective gear while treating Ebola victim Thomas Eric Duncan, health officials in Dallas say.
Duncan, who caught the virus in his native Liberia, died on Wednesday.
The current Ebola outbreak, concentrated in Liberia, Guinea and Sierra Leone, has resulted in more than 8,300 confirmed and suspected cases, and at least 4,033 deaths.
'Clearly a breach'
Dr Frieden said a full investigation would be conducted into how the infection had occurred
"Clearly there was a breach in protocol," he told US broadcaster CBS.
The CDC investigation, he told reporters, would focus on possible breaches made during two "high-risk procedures", dialysis and respiratory intubation.
Education and training of health workers would be stepped up, he said, and efforts would be made to reduce the number of staff treating Ebola cases.
Dr Daniel Varga, of the Texas Health Resource, said the health worker had worn a gown, gloves, mask and shield when providing care to Duncan during his second and final hospital admission.
Following a positive preliminary test for Ebola, follow-up tests on the infected health worker are due to be completed on Sunday.
Police are guarding the apartment complex where the woman lives in Dallas as decontamination work is carried out.
No details of her identity or position at the hospital have been given, in accordance with family wishes.
Flight from Monrovia
Duncan tested positive in Dallas on 30 September, 10 days after arriving on a flight from Monrovia via Brussels.
He had become ill a few days after arriving in the US, and went to the hospital in Dallas with a high fever.
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