Thomas Eric Duncan
Duncan, 42, died on Wednesday, two weeks after travelling to the US from Liberia, the west African country at the centre of the outbreak, where he was thought to have contracted the disease while accompanying a dying pregnant woman in a taxi to hospital.
Those caring for him had abided by Center for Disease Control and Prevention (CDC) guidelines which stipulate that full protective suits should be worn while treating Ebola victims.
The latest victim was in a group of 48 health care workers who were being monitored but were considered at low risk of developing the disease when they began to run a low-grade fever on Friday.
A preliminary test was positive for Ebola on Saturday evening, and although this has yet to be confirmed by a second test, officials said they were 'confident' it would be.
Duncan was sent home from hospital on the first occasion that he sought help, and was finally admitted four days later, by which time he was likely to have been highly contagious.
It was on this second visit to the hospital that the health worker came into contact with him.
Dr David Varga, the Chief Clinical Officer, said: 'This individual was following full CDC guidelines. We're very concerned.'
A nurse in Spain who treated a dying priest and went on to develop Ebola has admitted that she may have touched her face while removing her protective suit. The latest victim will be interviewed to see if they made a similar slip.
Officials said the latest victim was taken into isolation within 90 minutes of them arriving in hospital. They are in a stable condition and have requested 'total privacy'.
The fact that the Dallas worker caught Ebola despite taking apparently stringent precautions has alarmed the authorities, and is likely to create panic across America.
Dallas chief executive Judge Clay Jenkins, who is in charge of the civilian response to the outbreak, had already been shunned by his neighbours, with parents pulling their children out of his daughters' school.
Dr Kent Brantly at work in Monrovia (Reuters)
In August, when the first American Ebola victim, missionary Dr Kent Brantly, was flown back from Liberia for treatment in the US, Donald Trump, the billionaire property mogul, said he should have been forced to remain in Africa.
Officials said that the latest victim's apartment was being cleaned and their car had already been isolated and decontaminated.
A close relative had been put into quarantine, and their pet was being monitored.
Spanish authorities who euthanised a dog belonging to the nurse had faced protests.
