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, CDC: "At some point there was a breach in protocol"
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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.
In other developments
- The health authorities in Sierra Leone say they are now treating more Ebola patients in the capital Freetown than in the eastern districts of Kenema and Kailahun, where the first cases in the country were detected
- European health officials investigating how a nurse in Madrid caught Ebola told the BBC they believe it was simply the result of an accident and the risks to the wider population remain very low
- The UN special envoy on Ebola told the BBC the number of Ebola cases was currently increasing exponentially, but greater awareness would help contain the virus
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.
Duncan was admitted to Texas Health Presbyterian Hospital in Dallas
Police are guarding the home of the infected woman
A barrel labelled "biohazard" stands on a lawn outside the apartment complex of the infected health worker in Dallas
Staff at the Dallas hospital have been on alert for other cases after Thomas Duncan's death
US President Barack Obama discussed the Ebola response with Health and Human Services Secretary Sylvia Burwell by PHONE
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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A preliminary test shows a Texas health worker who treated Ebola victim Thomas Duncan before he died is also infected with the virus, US officials say.
"We knew a second case could be a reality, and we've been preparing for this possibility," said Dr David Lakey, commissioner of the Texas Department of State Health Services.
Mr Duncan, who caught the virus in his native Liberia, died at a Dallas hospital on Wednesday.
The health worker has not been named.
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.
Sent home by hospital
Mr 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 Texas Health Presbyterian Hospital in Dallas with a high fever.
But despite telling medical staff he had been in Liberia, he was sent home with painkillers and antibiotics.
Mr Duncan was later put into an isolation unit at the hospital but died despite being given an experimental drug.
It is not clear whether the health worker who tested positive came into contact with Mr Duncan when he first showed possible symptoms or when he was re-admitted to hospital.
The health worker reported a low-grade fever on Friday and was isolated and referred for testing, Texas health care services said in a statement.
"Health officials have interviewed the patient and are identifying any contacts or potential exposures," it added.
Tulip Mazumdar describes the protective measures taken by journalists covering the Ebola crisis
"People who had contact with the health care worker after symptoms emerged will be monitored based on the nature of their interactions and the potential they were exposed to the virus."
Some 50 people who had direct or indirect contact with Mr Duncan are reportedly already being monitored in case they develop symptoms.
Screening
More tests on the US health care worker are being carried out at the Centers for Disease Control and Prevention in Atlanta. If Ebola is confirmed, it would be the first time the infection is known to have spread within the US.
A nurse in Spain contracted the haemorrhagic fever while caring for patients who came from West Africa.
The arrival of Ebola in Texas prompted the US authorities to introduce screening of passengers from affected countries at airports, starting on Saturday at New York's JFK.
Passengers from those countries will have their temperatures taken and have to answer a series of questions.
With the numbers of those affected continuing to rise in West Africa, the UN special envoy on Ebola says he hopes that the outbreak can be brought under control within three months.
David Nabarro told the BBC the number of Ebola cases was currently increasing exponentially, but greater awareness would help contain the virus.
"I think we've got much better community involvement [now] which leads me to believe that getting it under control within the next three months is a reasonable target," he said.
Mr Nabarro described the accelerating increase of new cases as "quite frightening".
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