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Patients are at their most contagious when Ebola is in its terminal stages, inducing both internal and external bleeding, and profuse vomiting and diarrhoea - all of which contain high concentrations of infectious virus.

But the disease can also have a long incubation period of up to 21 days, meaning that people can be unaware for weeks that they are infected, and not feel or display any symptoms.

This, it seems, is what allowed the Liberian visitor Thomas Eric Duncanto to fly to the United States and spend several days there unaware that he was carrying the deadly virus, before being diagnosed and isolated.

In the European Union, free movement of people means someone unknowingly infected with Ebola could easily drive through several neighbouring countries before feeling ill and seeking help, and spend weeks in contact with friends or strangers before becoming sick enough to show up on airport scanners.

Jonathan Ball, a professor of molecular virology at Britain's Nottingham University said that even with exit screening at airports of affected countries, the long, silent incubation period meant "cases can slip through the net".

"Whilst the risk of imported Ebola virus remains small, it's still a very real risk, and one that won't go away until this outbreak is stopped," he said. "Ebola virus isn't just an African problem."

However, the chance of the disease spreading widely or developing into an epidemic in a wealthy, developed country is extremely low, healthcare specialists say.

According to the latest Ebola risk assessment from the European Centres of Disease Prevention and Control, which monitors health and disease in the region, "the capacity to detect and confirm cases...is considered to be sufficient to interrupt any possible local transmission of the disease early."

Gatherer cited Nigeria as an example of how Ebola can be halted with swift and detailed action.

Despite being in West Africa and being home to one of the world's most crowded, chaotic cities, Nigeria has managed to contain Ebola's spread to a total of 20 cases and 8 deaths, and looks likely to be declared free of the virus in coming weeks.

"Even if we have a worse case scenario where someone doesn't present for medical treatment, or..it's not correctly identified as Ebola, and we get secondary transmission, it's not likely to be a very long secondary transmission chain," he said.

"People aren't living in very crowded conditions (in Europe), so the disease doesn't have the same environment it has in a shanty town in Monrovia, where the environment is perfect for it to spread. It's a different matter in modern western cities with the very sanitised, sterile lives that we live."

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