WASHINGTON - The director of the US Centers for Disease Control and Prevention (CDC) said Sunday that a "breach in protocol" caused a health care worker in Texas to preliminarily test positive for Ebola after treating the first patient with the deadly virus in the United States.

If the test results are confirmed, the health care worker, reportedly a female nurse at Texas Presbyterian Hospital, would be the first known person-to-person transmission of Ebola in the United States.

"We are deeply concerned," CDC Director Tom Frieden told a press conference.

Frieden said the nurse had "extensive contact" on "multiple occasions" with Thomas Eric Duncan, the first person diagnosed with Ebola on US soil, who died earlier this week.

"We don't know what occurred in the care of the index patient -- the original patient in Dallas," said Frieden. "But at some point there was a breach in protocol, and that breach in protocol resulted in this infection."

According to the CDC, the health care worker reported a low grade fever and was referred for testing Friday. A preliminary test result done at a state public health laboratory in Austin Saturday showed that the worker contracted the virus.

Frieden said confirmation testing at a CDC laboratory will be done later Sunday.

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