A top US official is being criticised for announcing that a Dallas nurse was infected with ebola due to a protocol breach.
Dr Thomas Frieden made the claim over the weekend.
He has now called for an investigation into how the nurse, who has not been named, caught ebola while looking after Thomas Eric Duncan, the first patient to be diagnosed with the virus in the US.
The nurse, who was treating Mr Duncan at the Texas Health Presbyterian Hospital, was confirmed on Sunday to have been the first to contract the disease in the US.
The unnamed woman, who had been wearing full protective gear, reported having a "low-grade fever" on Friday and was then isolated, officials said.
Dr Thomas Frieden, from the Centres for Disease Control, said the hospital worker caught the disease because of a breach of care protocol during Mr Duncan's treatment.
A further 18 workers could also have been exposed, he said, and are being monitored.
Meanwhile, tests are being carried out on another suspected ebola sufferer in the US - hours after a female nurse was confirmed to have contracted the disease from a patient.
The man was put into isolation in Boston, Massachusetts, after recently returning from Liberia - one of the countries worst affected by the outbreak.
