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.
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