Ebola and the deadly Marburg virus also infect wild primates such as apes and some studies suggest humans contract the disease while handling infected carcasses.

The ultimate source is a mystery but evidence points to bats.

Dr Robert Swanepoel from South Africa's National Institute for Communicable Diseases, speaking at an Africa conservation conference in Madagascar on Monday, said: "There is bushmeat going into places like Paris and London, being smuggled in for the immigrant market, so an Ebola outbreak could happen there."

The Ebola and Marburg viruses are traditionally associated with central and west Africa, but that may change.

"Los Angeles is as close to Ebola as Kinshasa with air travel," Swanepoel said.

In the three decades since Ebola emerged and the four since Marburg was discovered, both viruses have killed 2,500 people - which may look deceptively insignificant compared to malaria or Aids.

"People ask why the fuss? For one thing, Ebola outbreaks are becoming more frequent and you have to ask why," Swanepoel said.

Widespread panic

The first recorded outbreaks of Ebola were in the former Zaire, now the Democratic Republic of Congo, and Sudan in 1976.

"Ebola and Marburg spark panic - people flee and, unlike Aids, they kill medical staff and paralyse medical services"

Dr Robert Swanepoel, South Africa National Institute for Communicable Diseases

Ebola victims bleed internally and externally from any opening in the body. Fatality rates are 90% or more and humans can still be infected by handling the corpse of someone who died from the disease.

The virus seemed to vanish after 1979, but re-emerged in the mid-1990s in Ivory Coast and Gabon, with several outbreaks occuring since in various countries. Marburg is also occurring more often.

"We think the increasing frequency is because of a population explosion and deforestation," Swanepoel said.

Africa's bushmeat trade is now run on an industrial scale, increasing the chances of spreading the disease.

The plague that killed a third of Europe's population in the mid-14th century also followed trade routes.