Fears are being raised that the worst ever outbreak of Ebola could spread to the UK after a man who flew from one of the affected countries - where the borders have only just been closed - later died.
The man died in the Nigerian city of Lagos, Africa's largest, which is far better connected to the outside world than the rural areas of Sierra Leone, Liberia and Guinea where the outbreak has been raging since March, killing around 670 people so far.
Patrick Sawyer, a Liberian citizen, collapsed on arrival at Lagos' airport and died in hospital. It is understood his flight had stopped over in Liberia.
Though air passengers are now being screened in the three affected countries, the virus incubates for between two and 21 days before the person shows any symptoms and the virus cannot always be detected in those newly infected.
Public Health England (PHE) told The Huffington Post UK that Sawyer's death was "a cause for concern" as it showed the disease had moved from one country to the other.
Dr Brian McCloskey, PHE's director of global health, said: "It's clear the outbreak is not under control.
"The continuing increase in cases, especially in Sierra Leone, and the importation of a single case from Liberia to Nigeria is a cause for concern as it indicates the outbreak is not yet under control. We will continue to assess the situation and provide support as required.
"The risk to UK travellers and people working in these countries of contracting Ebola is very low but we have alerted UK medical practitioners about the situation in West Africa and requested they remain vigilant for unexplained illness in those who have visited the affected area."
Dr McCloskey added: "It is important to stress that no cases of imported Ebola have ever been reported in the UK and the risk of a traveller going to West Africa and contracting Ebola remains very low since Ebola is transmitted by direct contact with the blood or bodily fluids of an infected person."
Professor Hugh Pennington, emeritus professor of bacteriology at the University of Aberdeen, told the Daily Mail: “If the disease gets going in Nigeria it would be cause for concern.
“Nigeria has close links with the UK and many other countries.”
He added: "This is a very big outbreak, and I get the feeling that whatever is being done to control it is not being done very well.
"Once you get a fairly large number of cases, it gets a momentum and becomes more and more difficult to control.
"This is now crossing international borders."
Liberia only closed its borders to combat the spread of the disease yesterday, meaning the only access to and from the country will be out of the national airport.
Dr Peter Walsh, an Ebola expert at Cambridge University, told The Daily Telegraph: “It’s possible someone infected will fly to Heathrow having infected other people sitting next to them or by using the toilet.
“This strain of Ebola is probably the second most deadly virus in the world after canine rabies. If you get canine rabies, you’re going to die, but we also have vaccines for that.
“This is worse than anthrax, but there are vaccines and treatments for anthrax, too.”
The disease has killed around 60% of those it has infected in the latest outbreak. In previous outbreaks, it has killed up to 90% of those infected.
Two Americans have been infected with the virus since the outbreak.
The second American, Nancy Writebol is a worker with an allied aid group SIM, or Serving in Mission, which runs the hospital where Samaritan's Purse has an Ebola care center on the grounds. He said she was in stable and serious condition.
Before the latest outbreak, 1,590 people were confirmed to have died from Ebola since it was first documented in 1976, out of 2,388 people infected, according to the World Health Organisation.