Amid growing concern, President Barack Obama cancelled a campaign trip to address the outbreak and vowed that his administration would respond in a "much more aggressive way" to Ebola cases in the United States.
Though it was not clear how the nurse contracted the virus, the case represented just the latest instance in which the disease that has ravaged one of the poorest corners of the earth — West Africa — also managed to find weak spots in one of the world's most advanced medical systems.
The second nurse was identified as 29-year-old Amber Joy Vinson. Medical records provided to The Associated Press by Thomas Eric Duncan's family showed she inserted catheters, drew blood and dealt with Duncan's body fluids.
Duncan, who was diagnosed with Ebola after coming to the U.S. from Liberia, died Oct. 8.
Kent State University in Ohio, where three of Vinson's relatives work, confirmed she was the latest patient.
Even though the nurse did not report having a fever until Tuesday, the day after she returned home, the director of the Centers for Disease Control and Prevention said she should not have boarded a commercial flight.
The nurse also knew before heading home that another nurse, Nina Pham, had been diagnosed with Ebola, and she had a slightly elevated temperature — 99.5 degrees, according to government officials.
While in Cleveland, she was contacted by health officials and told that her health would need to be more closely monitored for Ebola, the CDC said. Agency spokesman David Daigle said Wednesday evening that Vinson spoke to a CDC official before she boarded a plane back to Dallas, reporting her temperature was below 100.4 degrees and that she had no symptoms. The official, a contact tracer responsible for monitoring Vinson, cleared her to fly.
From now on, CDC Director Tom Frieden said, no one else involved in Duncan's care will be allowed to travel "other than in a controlled environment." He cited guidelines that permit charter flights or travel by car but no public transportation.
On its website, the CDC says all people possibly exposed to Ebola should restrict their travels — including by avoiding commercial flights — for 21 days.
Ebola patients are not considered contagious until they have symptoms. Frieden said it was unlikely that others on the plane were at risk because the nurse was not vomiting or bleeding.
Even so, the CDC is alerting the 132 passengers who were aboard Frontier Airlines Flight 1143 from Cleveland to Dallas-Fort Worth on Monday "because of the proximity in time between the evening flight and first report of illness the following morning." Officials are asking passengers to call the health agency so they can be monitored. The nurse flew from Dallas to Cleveland on Friday, Oct. 10.
Kent State said it was asking the workers related to Vinson to stay off campus for 21 days "out of an abundance of caution."
Her Ebola diagnosis was confirmed Wednesday.
The CDC's investigation suggests that health care workers were at highest risk from Sept. 28 to Sept. 30, the three days before Duncan was diagnosed. Both nurses who contracted Ebola worked on those days and had extensive contact with him when he had vomiting and diarrhea, Frieden said.
Medical records indicate that the workers wore protective equipment, including gowns, gloves and face shields during that time. The first mention in the records that they wore hazmat suits was on Sept. 30.
In his most urgent comments on the spread of the disease, Obama also warned that in an age of frequent travel, the disease could spread globally if the world doesn't respond to the "raging epidemic in West Africa."
The second nurse was transferred Wednesday to a special bio-containment unit at Emory University Hospital in Atlanta, where other Ebola patients have been treated successfully.
Pham will be monitored in Dallas to determine the best place for her care, Frieden said.
The CDC has acknowledged that the government was not aggressive enough in managing Ebola and containing the virus as it spread from an infected patient to a nurse at a Dallas hospital.
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 might also reveal which body fluids pose the greatest risk.
At the Dallas apartment complex where the second nurse lives, emergency responders in hazardous-materials suits began decontamination work before dawn Wednesday.
Police guarded the sidewalk and red tape was tied around a tree to keep people out. Notices handed out to neighbours advised of the diagnosis. Officials said she lives alone with no pets.
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 all right.
"It's a little shocking that it's right near me," he said.
Dallas city spokeswoman Sana Syed said a hazardous-materials crew cleaned common areas of the complex and that the state was sending a crew to clean the apartment.
In all, public-health officials are monitoring more than 100 people who might have been exposed to Ebola through Duncan — at least 76 of them at the hospital.
Associated Press medical writers Marilynn Marchione and Mike Stobbe and AP reporters Martha Mendoza, Maud Beelman, Matt Sedensky and Alex Sanz in Dallas also contributed to this report.