An unnamed nurse who treated Thomas Eric Duncan while he was confined at the Texas Health Presbyterian Hospital in Dallas was found positive of the Ebola virus. The infection marks the first case of Ebola transmission in the U.S.

Duncan became the first Ebola fatality in the U.S. when he died last week but even after his death, Ebola continues to be a concern in the country particularly now that a healthcare worker was confirmed infected by the deadly virus. Health officials said that the unnamed woman had close and frequent contact with Duncan while he was treated.

It is not yet clear how the healthcare worker was infected. Hospital officials said that the nurse had been wearing full protective gear recommended by the CDC including gowns, gloves, masks and shields while treating Duncan but she still contracted the disease.

The woman started exhibiting symptoms on Friday and was isolated 90 minutes after her arrival at the hospital. Result of Ebola test came out the next day and this revealed she is positive of Ebola.

U.S. Centers for Disease Control and Prevention (CDC) director Thomas Frieden said that there was a breach in protocol and this professional lapse may have caused the infection of other healthcare workers at the Texas Health Presbyterian Hospital as well.

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

The CDC director also said that they are evaluating other healthcare workers for potential exposure as there is possibility that other workers at the hospital may have also been exposed to the deadly virus.

"Even a single breach can result in contamination, and one of the areas that we look at closely are things like how you take off the gear that might be affected or contaminated," Frieden said.

As per Frieden, it is possible that new cases of Ebola will emerge in the coming days. "This is because the healthcare workers who cared for this individual may have had a breach of the same nature of the individual who appears now to have a preliminary positive test."

Health officials are also trying to determine the number of people that the patient had in contact with. Frieden said that the infected nurse had close contact with one person when she could already transmit the disease and this person is currently being monitored.