The Ebola outbreak in West Africa is by a long way the worst ever. It has infected at least 8,000, killed more than 3,000 and has led to the breakdown of the healthcare systems of entire states in the worst affected countries: Guinea, Liberia and Sierra Leone.
Scientists believe that the first case in the current outbreak was a two-year-old child in southern Guinea, in December last year. But the history of the Ebola virus goes back further.
It was first identified in Zaire (modern day Democratic Republic of the Congo) in 1976. The first victim was a schoolmaster in the village of Yambuku in the north of the country. The disease got its name from the nearby Ebola River, a tributary of the Congo River. That first outbreak infected at least 318 people and killed 88 per cent of the people affected.
Another simultaneous outbreak, in Sudan, infected nearly 300 people, with a mortality rate of around 50 per cent.
Since then there have been several outbreaks, but none anywhere near as devastating as the current one.
It is believed that the virus originates in fruit bats. It circulates in populations of wild animals including gorillas, fruit bats, monkeys, antelopes and even porcupines.
While it is not known how the first ever victim – or the little girl who was the first victim of the current outbreak – were infected, there are many potential ways Ebola could pass from animals to humans.
It is transmitted through contact with bodily fluids. Eating fruit collected from the forest floor, that an infected fruit bat had bitten, could spread it. So could contact with the blood of an infected animal that had been butchered for bush meat.
See the Ebola outbreak mapped
See the Ebola outbreak mapped
1/7 25 March 2014
This outbreak of the Ebola virus first emerged in the Guéckédou region of Guinea, at a crossroads with both Liberia and Sierra Leone
2/7 31 March
On 31 March the WHO confirmed the outbreak was now international, spreading first into Liberia's northern-most Lofa region
3/7 27 May
The virus spread to Sierra Leone at the end of May - just as agencies were hoping the worst was over
4/7 27 July
In Sierra Leone the virus boomed, and then it spread to Nigeria when the Liberian diplomat Patrick Sawyer flew from Monrovia to Lagos
5/7 9 August
The Nigeria cases sparked fears around the world, and there have now been deaths in Spain and Saudi Arabia involving people who had travelled to West Africa. The numbers of cases continue to rise
6/7 17-20 September
In mid-September, Senegal confirmed its first case linked to the Ebola outbreak, a development the WHO described as a top priority emergency. Numbers of cases continued to grow exponentially in Guinea, Sierra Leone and Liberia, as experts warned they could number one million by January if not contained
7/7 8 October
Two cases of Ebola have now been reported in the US and Europe - the first times the virus has been contracted among health workers outside Africa
Monkeys, apes and antelopes are commonly eaten in the areas where the outbreak began. If someone were handling the raw meat and had an open cut on their hand, that could transmit the virus.
Once the virus is in human circulation, it becomes far harder to contain. Health care workers have been at particular risk because they have come into direct, close contact with victims.
Traditional burial ceremonies among many of the communities affected involve direct contact with the body of the dead, and this is believed to have been a major factor in the early spread of the virus, before public safety messages began to get through to people.
The virus can also be transmitted through sex. The WHO says that even men who have recovered from the virus can still transmit it through their semen for up to seven weeks after recovery.