The Texas nurse who contracted Ebola while caring for a dying Liberian man repeatedly visited his room from the day he was admitted to the intensive care unit until the day before he died, medical records show.
Nurse Nina Pham and other health care workers wore protective gear, including gowns, gloves, masks and face shields - and sometimes full-body suits - when caring for Thomas Duncan.
But 26-year-old Ms Pham became the first person to contract the disease within the United States.
Health officials yesterday urged US hospitals to "think Ebola" and launched a review of procedures for treating infected patients, while the World Health Organisation called the outbreak "the most severe, acute health emergency seen in modern times".
Ebola has killed more than 4,000 people, mostly in the West African countries of Liberia, Sierra Leone and Guinea, according to WHO figures published last week.
Ms Pham's family told WFAA-TV in Dallas that she was the health care worker with Ebola.
She was among about 70 hospital staffers who were involved in Mr Duncan's care when he was in hospital, according to the records.
The Texas Christian University nursing school graduate was monitoring her own temperature and went to the hospital on Friday night as soon as she discovered she was running a low fever. She is in isolation and in stable condition, health officials said.
By yesterday evening, she had received a transfusion of plasma from Kent Brantly, a Texas doctor who survived the virus.
Dr Brantly, an aid worker in Liberia, was flown back to the US and treated with the experimental drug ZMapp at Emory University Hospital in Atlanta.
Another American doctor who contracted Ebola in Liberia, Phil Sacra, received two blood transfusions from Dr Brantly as part of a treatment programme at the Nebraska Medical Centre that allowed him to recover from the virus.
Public health authorities have since intensified their monitoring of other Dallas hospital workers who cared for Mr Duncan.
Centres for Disease Control and Prevention Director Tom Frieden said he would not be surprised if another hospital worker who cared for Mr Duncan becomes ill because Ebola patients become more contagious as the disease progresses.
Ms Pham's name appears frequently throughout the hundreds of pages of documents in Mr Duncan's medical records, provided by his family.
Mr Frieden has said a breach of protocol led to the nurse's infection, but officials are not sure what went wrong. Ms Pham has not been able to point to any specific breach.
The CDC now is monitoring all hospital workers who treated Mr Duncan and planned to "double down" on training and outreach on how to safely treat Ebola patients, Mr Frieden said.
Health officials have relied on a "self-monitoring" system when it comes to US health care workers who care for isolated Ebola patients and wear recommended protective equipment.
They expect workers to report any potential exposures to the virus and watch themselves for symptoms.
Besides the workers, health officials continue to track 48 people who were in contact before Mr Duncan was admitted to the hospital and placed in isolation.
They are monitoring one person the nurse was in contact with while she was in an infectious state.
None has exhibited symptoms, Mr Frieden said.
The case involving Ms Pham raised questions about assurances by American health officials that the disease will be contained and any US hospital should be able to treat it.
Mr Duncan, who arrived in the US from Liberia on September 20, first sought medical care for fever and abdominal pain on September 25.
He told a nurse he had travelled from Africa, but he was sent home. He returned on September 28 and was placed in isolation because of suspected Ebola.
Among the things the CDC will investigate is how the workers took off protective gear, because removing it incorrectly can lead to contamination.
Investigators will also look at dialysis and intubation - the insertion of a breathing tube in a patient's airway. Both procedures have the potential to spread the virus.
Health care workers treating Ebola patients are among the most vulnerable, even when wearing protective gear.
More than 370 health care workers in West Africa have fallen ill or died since the epidemic began earlier this year.
Officials said there was a dog in the Texas nurse's apartment that has been removed to an undisclosed location for monitoring and care.
They do not believe the pet shows any signs of Ebola. A dog belonging to an infected Spanish nurse was euthanised, drawing thousands of complaints.