Initially health officials wanted to trace only a few hundred passengers on two planes which had carried victim Patrick Sawyer, 40.
But – as Cabinet ministers held an emergency Cobra meeting in London – the search was widened to find up to 30,000 people who could be hosting the organism which kills 90% of sufferers.
The list includes anyone at one of four airports visited by American dad-of-three Sawyer, and those in contact with him in Nigeria’s capital Lagos, home to 17 million, where he died five days ago.
Mike Noyes, of Action Aid, said: “This is the worst Ebola outbreak the world has ever seen.
"The most worrying thing is not just the numbers of people dying, but how long it is going on for.
“Most outbreaks last six weeks to two months. This one began in February and is speeding up.
“If anyone could answer the question ‘Why?’, we might be able to stop it. Instead, the reach of the spider web of infection is growing.”