US health officials are seeking 132 people who flew on a plane with a Texas nurse on the day before she came down with symptoms of Ebola.
The second person infected in the US, Amber Vinson, 29, fell ill on Tuesday.
Both she and nurse Nina Pham, 26, had treated Liberian Thomas Eric Duncan, who died a week ago in Dallas.
A nurses' union has said those treating Duncan were not given full protection and had parts of their skin exposed
More than 70 healthcare workers who may have come in contact with him at the hospital are being monitored for symptoms, the hospital's director has said.
Meanwhile, the UN's Ebola mission chief says the world is falling behind in the race to contain the virus, which has killed more than 4,000 in West Africa.
On Wednesday, the US Centers for Disease Control and Prevention (CDC) said it wanted to interview the passengers on Frontier Airlines flight 1143 from Cleveland, Ohio, to Dallas, Texas on 13 October.
It said it was taking the measure "because of the proximity in time between the evening flight and first report of illness the following morning".
Both Ms Vinson and Ms Pham treated Duncan early in his stay at Texas Health Presbyterian Hospital in Dallas when he had "extensive production of body fluids", said CDC director Tom Frieden.
A national nurse union told reporters on Wednesday the health workers treating Duncan had not been properly protected and called for all health workers treating Ebola patients to receive full protective suits and training from hospitals.
Union director RoseAnn DeMoro said staff treated him for days without the necessary protective gear, and hazardous waste was allowed to pile up to the ceiling.
In this photo posted on Twitter by the City of Dallas Public Information Managing Director Sana Syed, members of Dallas Fire-Rescue Haz Mat Unit tape off the door at the The Village Bend East apartment of a second healthcare worker who has tested positive for Ebola, in Dallas, on 15 October 2014
Dallas officials tape off the apartment of the second nurse who tested positive for Ebola
The CDC has appointed a "site manager" in Dallas to standardise the protective equipment and supervise the method of putting it off and on.
Ms Vinson flew to Cleveland on 10 October, even though she was being monitored for signs of Ebola and therefore should not have flown on a commercial aeroplane, Dr Frieden said.
When Ms Vinson returned from Ohio, she was not showing symptoms of the disease, the crew has told CDC investigators.
On the morning of 14 October, Ms Vinson came down with a fever and was isolated within 90 minutes. Her diagnosis was announced early on Wednesday.
One of the ill women is to be transferred to Emory University hospital in Atlanta, which oversaw the recovery of two US aid workers who had caught the disease in Africa.
Mr Duncan, who was the first person to be diagnosed in the US with Ebola, started showing symptoms of the disease just days after he arrived in Texas from Liberia, where he contracted it.
An initial set of 48 people who were in contact with him before he was admitted to hospital are nearing the end of the window in which they could develop an Ebola infection.
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