The US Centers for Disease Control and Prevention has confirmed that a Texas health care worker has tested positive for Ebola, the first case contracted on US soil.
"This development is understandably disturbing news for the patient, the patient's family and colleagues and the greater Dallas community," the CDC said in a statement.
"The CDC and the Texas Department of State Health Services remain confident that wider spread in the community can be prevented with proper public health measures."
Earlier, health officials said the health care worker, who had treated a Liberian man who died of Ebola, was wearing full protective gear but was infected with the virus anyway.
CDC director Dr Thomas Frieden said a breach in safety protocols, possibly while removing protective gear, may have caused the health care to contract the virus.
He warned that all of the Dallas healthcare workers who helped care for Thomas Eric Duncan, who died last week at the Texas Health Presbyterian Hospital, were potentially exposed to the virus.
"Another (area) that we'll be looking at closely in the investigation is the interventions that were done to try desperately to keep (Duncan) alive," Dr Frieden said.
"This included dialysis and intubation. These are two procedures which can result in the spread of infectious material," Dr Frieden said.
He also said that other healthcare workers who came into contact with Mr Duncan could be at risk of contracting the disease.
"If this individual was exposed, which they were, it is possible that other individuals were exposed," he told a news conference.
"The level of her symptoms and indications from the test itself suggest that level of virus that she has was low," Dr Frieden said of the infected worker.
He said there was one person who may have had contact with the infected health worker when she could possible transmit the disease and that person is being monitored.
The worker's family has requested total privacy.
President Barack Obama wants federal authorities to take additional steps to ensure the US medical system is prepared to follow correct protocolsin dealing with Ebola following news of an apparent breach in Dallas, the White House said in a statement.
Mr Obama said federal authorities should "take immediate additional steps to ensure hospitals and healthcare providers nationwide are prepared to follow protocols should they encounter an Ebola patient."
Earlier, Dan Varga, the chief clinical officer for Texas Health Resources, said officials were "very concerned" that the worker had apparently contracted the virus despite following safety protocol.
"This individual was following full CDC precautions," he said, noting that the gear would have included a mask, gown, and gloves.
Mr Duncan arrived from Liberia on 20 September, and began feeling sick about four days later.
He was initially sent home from a Dallas hospital on 25 September when he sought treatment for a fever an abdominal pain, even though he also told staff of his recent travel to Africa.
He returned by ambulance on 28 September to the same hospital.
Mr Duncan was estimated to have come in contact with about 100 people, and officials narrowed that pool down to 48 in the days after his infection was diagnosed.
The health care worker was not in the high risk group of people, Dr Varga said, but had been asked to check his or her temperature twice daily and report back if any signs of fever, according to a protocol approved by the CDC.
"This health care worker had been under the self-care monitoring regimen recommended by the CDC," said Dr Varga.
The person is the second case of Ebola diagnosed in the United States, as an epidemic spreads in West Africa that has killed more than 4,000 people since the beginning of this year.
Patient with Ebola symptoms in Boston hospital
A patient in Massachusetts who recently returned from Liberia and was displaying symptoms of Ebola was transferred from a medical clinic to a Boston hospital, the hospital said.
The patient reported to the Harvard Vanguard Medical Associates hospital in Braintree, Massachusetts, complaining of a headache and muscle aches, said Ben Kruskal, a physician and chief of infectious disease, in a statement.
The patient has not been confirmed to have the deadly virus.
"Out of an abundance of caution we immediately notified authorities and the patient was securely removed from the building and put into an ambulance now headed to Beth Israel Deaconess Medical Center," Dr Kruskal said.
Dr Kruskal said the Braintree building was closed briefly but reopened. "We are working closely with the Department of Public Health who will determine next steps," he said.
Fire and ambulance services responded to the hospital and put an Ebola protocol in place, said William Cash of the Braintree Fire Department. He said the patient is a male.
Signs of hope for Spanish nurse with Ebola
Meanwhile, Spanish health authorities said there were signs of hope for a nurse infected with Ebola in Madrid as the levels of the virus in her body were diminishing, though they also said she remained in a serious condition.
Teresa Romero, 44, became the first known person to become infected with Ebola outside Africa during the current outbreak after she cared for two infected priests repatriated to Spain for treatment. The priests later died.
"The patient appears to be in a stable condition ... There are some signs that could give us cause for some hope," Fernando Simon, a high level official at Spain's Health Ministry, told a news conference.
"There are high hopes that the infection is starting to come under control," he said, adding she was not yet out of danger.
"We have to be very cautious," Mr Simon said.
Ms Romero has spent the past week in hospital and had taken a turn for the worse a few days ago.
Yesterday evening, however,she was conscious and was responding to hospital staff after being given antibodies from previously infected patients.
Spain is still investigating how exactly Ms Romero could have contracted the disease, amid recriminations over the government's handling of the case.
Some Spanish media have said Ms Romero has been treated with ZMab, a combination drug made by Canada-based company DefyrusInc.
ZMab is one of the agents used to make ZMapp, an experimental treatment which has been used on some Ebola sufferers, a number of whom survived.
Spanish authorities have not confirmed the reports. A health source said ZMab was available in Spain but could not confirm whether it had been used in Ms Romero's case.
Fifteen people, including Romero's husband, are being monitored for signs of Ebola in a special isolation unit of Madrid's Carlos III hospital. None have so far shown any symptoms.
