US health authorities are trying to track down passengers who flew alongside a nurse infected with Ebola on 13 October.
Amber Vinson, who was one of the staff involved in treating Ebola sufferer Thomas Duncan at the Texas Health Presbyterian Hospital in Dallas, went aboard a commercial flight from Ohio to Texas after the Centers for Disease Control and Prevention (CDC) confirmed that it was safe for her to do so.
At the time, she had not been tested for Ebola but had called the CDC to report a slight fever before boarding the plane.
Because her fever was 99.5 degrees (37.5C) – slightly below the threshold of being a high risk – the person she spoke to on the phone said it was safe for her to fly.
Vinson was considered in the category of "uncertain risk", which is a lower level, because it was believed that she had worn all the necessary personal protective equipment while treating Duncan.
Nina Pham, another nurse who treated patient Thomas Duncan, had already fallen ill with the disease by that point.
On Wednesday, two days after the flight, Vinson tested positive for Ebola.
According to CDC spokesman David Daigle, Vinson was not showing any other symptoms of the disease at the time, adding that Ebola patients are not considered contagious until they show symptoms.
A search has been launched by the health authorities to find the other 132 passengers who were on Vinson's flight.
Meanwhile, the CDC is now investigating the way Texas Health Presbyterian Hospital used protective clothing during the treatment of Duncan, who came to the emergency room with Ebola on 28 September.
"Some of the forms of PPE [personal protective equipment] did allow exposure of some parts of the skin," said Tom Frieden, head of the CDC.
"The key is the adherence to protocol and the problems that we have identified had to do with using PPE in unfamiliar ways," he added, in reference to the fact that staffers may not have been familiar enough with how to don and discard the protective gowns, gloves and masks.