"If this one individual was infected, and we don't know how within the isolation unit, then it is possible that other individuals could have been infected as well," Frieden said of the nurse. "So, we consider them to potentially be at risk, and we're doing an in-depth review and investigation."
On Sunday, health officials said a breach of infection control led to the nurse getting Ebola and revealed that even in the United States, with the best conditions and protective gear available, mistakes can happen that expose more people to the deadly virus.
The nurse was said to have worn full personal protective equipment while caring for Duncan. She doesn't know how she became infected, something officials are investigating.
Frieden said health officials have identified the one and only contact the nurse had during the period she was potentially infectious. That individual is being monitored, but has shown no symptoms of Ebola.
Frieden said there were 48 potential contacts with Duncan —10 known to have had contact with him and 38 who may have had contact. All those contacts have been monitored daily, but none has developed symptoms.
Of the six Ebola patients treated in the U.S. before the health worker's case, Duncan was the only one not treated at one of the specialized units in four hospitals around the country set up to deal with high-risk germs.