The Ebola virus must hide somewhere between outbreaks, but no-one has yet discovered where - it may be in monkeys or some other animal host, or it may be in healthy human carriers. Once an outbreak does occur the virus is spread from person to person via blood and bodily secretions.

It can possibly infect a new victim via a cut or scratch on the skin, and can certainly do so through an injection with an unsterile needle. It can also be spread sexually. A small number of people, although infected, do not develop serious disease. Most others, however, are susceptible to the virus and will fall ill.

The incubation period is between four and 16 days. Once inside the body the virus multiplies and invades all the major organs. There is a high fever, headache and joint pains, vomiting and diarrhoea. The characteristic haemorrhaging or bleeding is caused by a process called DIC - disseminated intravascular coagulation.

Through this, the virus provokes massive clotting within the organs, and this leads paradoxically to bleeding because it uses up all the clotting factors in the blood. The clotting itself causes failure of the liver and kidneys, and this coupled with the haemorrhaging almost invariably leads to a fatal outcome.