There is an ongoing outbreak of the Ebola virus in Guinea in west Africa. Medical experts are concerned about it appearing in a densely populated area - the capital city of Conakry - and into neighbouring countries. Ebola is one of the most virulent diseases known to man and can kill up to 90% of the people it infects in a very short time period. But what causes it? How does it spread from human to human? And why is it unlikely to spread across continental borders?