A second Dallas hospital worker who provided care for the first Ebola patient diagnosed in the U.S. has tested positive for the disease, pointing to lapses beyond how one individual may have donned and removed personal protective garb.

It's not clear how the second worker contracted the virus. Authorities declined to say what position she holds at Texas Health Presbyterian Hospital or the type of care she provided to Thomas Eric Duncan, who was diagnosed with Ebola after coming to the U.S. from Liberia. Duncan died Oct. 8.

Officials have said they also don't know how the first health worker, a nurse, became infected. But the federal Centers for Disease Control and Prevention said "an additional health care worker testing positive for Ebola is a serious concern."

"What happened there (in Dallas), regardless of the reason, is not acceptable. It shouldn't have happened," Anthony Fauci, director of the Institute of Allergy and Infectious Diseases of NIH, said on MSNBC Wednesday.

A Liberian Ebola patient who died at Texas Health Presbyterian Hospital was left in an open area of the ER for hours, and the nurses treating him worked for days without proper protective gear and faced constantly changing protocols, the largest U.S. nurses' union says. (LM Otero/The Associated Press)

The CDC and Frontier Airlines said Wednesday that the second health care worker had travelled from Cleveland to Dallas-Fort Worth on Monday aboard flight 1143. Health officials are alerting the 132 passengers who were on the same flight.

Infected Ebola patients are not considered contagious until they have symptoms. The airplane's crew said the nurse had no symptoms of Ebola during the flight. But the CDC is asking the passengers to call the health agency so they can be monitored.

The worker was monitoring herself for symptoms, Dallas County Judge Clay Jenkins said. The unidentified woman reported a fever Tuesday. She was in isolation within 90 minutes, Jenkins said.

"We are looking at every element of our personal protection equipment and infection control in the hospital," said Dr. Daniel Varga, chief clinical officer for Texas Health Resources, which operates Texas Health Presbyterian Hospital Dallas.

Dr. Tom Frieden, head of the CDC, has acknowledged that the government wasn't aggressive enough in managing Ebola and containing the virus as it spread from an infected patient to a nurse at a Dallas hospital.

Centers for Disease Control (CDC) Director Tom Frieden admitted Tuesday that mistakes were made in communicating protocols for the handling of patients with Ebola. (David Tulis/Atlanta Journal Constitution/AP)

"We could've sent a more robust hospital infection control team and been more hands-on with the hospital from day one about exactly how this should be managed," he said Tuesday.

The second case may help health officials determine where the infection control breach is occurring and make practices safer for health workers everywhere. For example, if both health workers were involved in drawing Duncan's blood, placing an intravenous line or suctioning mucus when Duncan was on a breathing machine, that would be recognized as a particularly high-risk activity. It also might reveal which body fluids pose the greatest risk.

The CDC was conducting confirmatory testing of a preliminary Ebola test conducted late Tuesday at a state public health laboratory in Austin, Texas, which came back positive.

Emergency responders in hazardous materials suits began decontamination work before dawn Wednesday at the Dallas apartment complex where the second hospital worker lives. Officials said she lives alone with no pets.

Notices handed out to neighbours advise that "a health care worker who lives in your area has tested positive for Ebola."

Ryan Fus, 24, who lives in the same building as the blocked-off apartment, said police knocked on his door before 6 a.m. to notify him and make sure he was doing OK.

"It's a little scary. It's a little shocking that it's right near me," he said. "But I'm not afraid or anything like that. I'm not gonna run away."

Dallas city spokeswoman Sana Syed said a hazardous materials crew has finished cleaning common areas of the complex and that the state was sending a crew to clean the actual apartment.