Health care worker who cared for the Ebola patient who died last week tested positive for the deadly virus, sending health officials scrambling Sunday to find the "breach in protocol" that resulted in her infection.

While the new Ebola patient was not publicly identified, officials said that she was a nurse who had helped treat Mr. Duncan at a hospital here and that she may have violated safety protocols.

It was the first confirmed instance of Ebola being transmitted in this country. Officials expanded the pool of people they had been monitoring, because the nurse had not been among the 48 health care workers, relatives of Mr. Duncan and others whom they were evaluating daily.

The worst Ebola epidemic in history, which has already killed over 4,000 people, broke out in West Africa in March. Since then, medical officials have been sounding the alarm that the disease may spread like wildfire around the planet.

Here's a brief history of Ebola firsts in the U.S.

The C.D.C. said it would conduct a nationwide training conference call on Tuesday for thousands of health care workers to ensure they would be fully prepared to treat a patient with Ebola.

To stop Ebola, US must be able to diagnose it quickly

“The care of Ebola patients can be done safely, but it’s hard to do it safely,” Dr. Thomas R. Frieden, director of the C.D.C., told reporters Sunday. “Even a single, inadvertent innocent slip can result in contamination.”