The patient is a nurse at Texas Hea

The patient is a nurse at Texas Health Presbyterian Hospital in Dallas, an official who is familiar with the case told CNN.

The Centers for Disease Control and Prevention in Atlanta conducted testing on the blood sample, and confirmed the results Sunday, marking the first known transmission of Ebola in the United States and the second diagnosis in the country.

She was involved in Duncan's care after he was placed in isolation -- his second trip to the hospital after coming to the United States from Liberia -- said Dr. Tom Frieden, the director of the CDC.

The nurse is in stable condition, Texas Health Resources chief clinical officer Dan Varga said. Duncan, the first person to be diagnosed with Ebola in the United States, died Wednesday.

The nurse had "extensive contact" on "multiple occasions" with Duncan, Frieden said.

"At some point, there was a breach in protocol, and that breach in protocol resulted in this infection," he said at a news conference Sunday. "The (Ebola treatment) protocols work. ... But we know that even a single lapse or breach can result in infection."

Also, Varga said that someone who is a "close contact" of the nurse has been "proactively" placed in isolation.