The Prime Minister took part in a conference call with the US president, as well as the leaders of Germany, France and Italy, after US health chiefs put out an alert to 132 passengers who were on board a flight with the second person to contract Ebola in the US.
Amber Vinson, 29, a nurse at Texas Health Presbyterian Hospital, travelled on a Cleveland to Dallas flight on 13 October – the day before she began showing symptoms.
Both she and fellow nurse Nina Pham treated Liberian man Thomas Eric Duncan, who died last week after becoming the first person diagnosed with Ebola in the US.
A senior Dallas official said there was a “real possibility” of further cases, and contingencies were being prepared.
Ms Vinson, who was diagnosed on Tuesday, lives alone. It is not clear how many people she may have been in contact with.
She was put into isolation within 90 minutes of her temperature spiking.
Even though the nurse did not report having a fever until she returned home from her flight, she should not have boarded a commercial airliner after learning that another nurse had been diagnosed with Ebola, government officials said.
Tom Frieden, director of the US Centres for Disease Control and Prevention (CDC) said no-one else involved in Mr Duncan’s care will be allowed to travel “other than in a controlled environment”.
Infected Ebola patients are not considered contagious until they have symptoms. Mr Frieden said it was unlikely that other passengers or airline crew members were at risk because the nurse did not have any vomiting or bleeding.
Even so, the CDC is alerting the 132 passengers aboard the Frontier Airlines Flight 1143 “because of the proximity in time between the evening flight and first report of illness the following morning”.
“We are looking at every element of our personal protection equipment and infection control in the hospital,” added Dr Daniel Varga, chief clinical officer for Texas Health Resources, which operates Texas Health Presbyterian Hospital, Dallas.
Amid escalating concerns about the disease, Mr Cameron held the video conference with Mr Obama and French, German and Italian leaders.
The White House confirmed that a trip by Mr Obama to New Jersey and Connecticut had been postponed to allow the president to take part in the high-level talks.
Earlier, during Prime Minister’s Questions in the House of Commons, Mr Cameron pledged the UK government “will do everything we can to keep this country safe”.
He said: “There will be another Cobra meeting chaired today by the Foreign Secretary. I’ll be chairing one tomorrow. We’re looking at all these issues about where people are arriving, and co-operating properly with all the devolved authorities.
“It is worth stressing there are no direct flights from Liberia, Sierra Leone or Guinea into the United Kingdom, so we’re talking about people who come here indirectly, and that is why it’s so important we put in place the screening processes, starting at Heathrow but to be rolled out more as the days go by.
“I’m absolutely convinced that we will do everything we can to keep this country safe.
I will make sure that proper liaison not only with Northern Ireland but also with the Republic takes place.”