For every three who survive, seven will die

Few people have been infected with the deadly Ebola virus and have lived to tell the tale. For every three who survive an outbreak, there can be up to another seven who end up dead from it.

Few people have been infected with the deadly Ebola virus and have lived to tell the tale. For every three who survive an outbreak, there can be up to another seven who end up dead from it.

The symptoms begin with headaches, fever, muscle pains, fatigue and diarrhoea. They can take days or even weeks to appear after the initial infection, which usually occurs by close contact with another infected person.

Within a week, sometimes days, the patient goes rapidly downhill. The alimentary canal, from mouth to anus, becomes a particular target with the patient producing black vomit and bloody stools.

Richard Preston, author of The Hot Zone, a bestselling book on Ebola outbreaks, describes in lurid detail how the virus begins to digest the inner fabric of the body, turning organs into a "digested slime".

"Your mouth bleeds, and you bleed around your teeth, and you may have haemorrhages from the salivary glands - literally every opening in the body bleeds, no matter how small," he writes.

"The surface of the tongue turns brilliant red and then sloughs off, and is swallowed or spat out. It is said to be extraordinarily painful to lose the surface of one's tongue."

There is no cure for Ebola and the only treatment is to try to keep the body comfortable and prevent dehydration. Barrier nursing, where there is no direct contact with the patient or their body fluids, is the only way of preventing further spread within a hospital.

The Ebola virus is one of a group of hair-like "filoviruses" that can cause haemorrhagic fever, when the breakdown of the tissues and failure of the blood-clotting system causes extensive internal bleeding.

It is named after the Ebola river in Zaire, now the Democratic Republic of the Congo, where the first known outbreak was recorded in 1976.

People who come into contact with the body fluids of an infected person run an exceptionally high risk of becoming infected, which is why family members and healthcare workers are often among the victims of an outbreak.

Tracing the ultimate source of an epidemic has proved difficult. Monkeys and apes can also be infected with the virus and, like humans, they die as a result. Scientists, however, are not sure which wild animal is the natural "reservoir" of the disease.

After the last outbreak, in the city of Kikwit in Zaire, when 244 people died out of 316 cases, virologists launched an extensive survey of native species to try to locate the source. The American Centers for Disease Control in Atlanta, Georgia, collected blood and tissue specimens from 3,000 vertebrates and 30,000 arthropods but have yet to identify which animal is the natural host.

Although contact with body fluids is the normal mode of transmission, reusable needles and syringes have also been effective vectors for the virus.

One fear is that the Ebola virus could mutate to a form that is transmissible through the air, being carried on water droplets much like the flu virus. There is no evidence that this has ever occurred.