The killer virus continues to claim lives in Africa and has now spread beyond it.Matthew Partridge looks at the threat it poses.
What are the latest developments?
The number of Ebola cases continues to grow in Africa, with the World Health Organisation now putting the death toll at over 3,000. Save the Children estimates that there are five new cases every hour in Sierra Leone, and warns that this could soon double to ten an hour if things are left unchecked.
Meanwhile, there are growing fears that the disease is spreading outside west Africa. Last week saw the first case of Ebola in the United States in someone who developed the disease after arriving (as opposed to being evacuated to an American hospital). Over 100 people who were exposed to the patient have been isolated.
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Is Ebola a major threat to developed countries?
At the moment experts still think that it could be contained relatively easily. While Ebola has a high mortality rate of up to 70%, it is hard to catch the disease from an infected person. At the moment, official guidance suggests that you need direct contact with their blood or bodily fluids, so sitting next to them, even on a crowded plane, won't be sufficient.
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Additionally, people only become infectious at the same time they start to develop symptoms, such as fever, diarrhoea and vomiting. This means that it should be easy to identify infected people and isolate them. Past outbreaks have suggested that identifying 70% of infected people promptly should be enough to bring the epidemic under control.
In fact, it has already been successfully contained in Nigeria, which experienced an outbreak in July. The only reason why Liberia and Sierra Leone are having problems is that they are two of the poorest countries in the world.
So the problem will be easily contained?
Not quite. While the disease is hard to pick up, there are fears that the virus could mutate to become much more contagious, so that it could be transmitted through the air. There is already evidence that this can happen in some non-human species.
For instance, in a study carried out two years ago, an infected pig passed the disease to a monkey nearby without any direct contact. Experts have pointed out that a mutation enabling this to take place in humans is unlikely.
However, the UN secretary-general's special representative thinks that "the longer it moves around in human hosts in the virulent melting pot that is west Africa, the more chances increase that it could mutate".
Is globalisation responsible for the spread of diseases?
Throughout history, increasedtravel has led to the spread of new diseases. Historians believe that syphilis was brought back by sailors from the New World, and was then spread through Europe by armies.
There issome evidence that increased tradeand travel has speeded up this process. As The Guardian's Nicky Woolf putsit, "as our cities get bigger and international travel becomes easier,the risks involved in an outbreak grow ever higher".
However, there is evidence that globalisation can also have benefits. Just as disease can spread more quickly from the periphery to the centre, resources and technology such as vaccines and new drugs can flow outward.
By raising living standards, globalisation can also increase the overall standard of living, allowing countries to improve their public health systems.
Would a travel ban help?
Some politicians in America are already calling for a totaltravel ban between America and affected countries. However, experts are sceptical about whether this would work. Atul Gawande in The New Yorker argues that "even if travel could be reduced by 80% itself a feat models predict that new transmissions would be delayed only a few weeks".
He also points out that "health-care workers who have fallen ill would not be able to get out for treatment, and the international health personnel needed to quell the outbreak would no longer be able to go in".
Should we be worried?
Several experts have argued that, despite the headlines, Ebola is a relatively minor threat. The Guardian's James Ball (writing in August) noted that "since the Ebola outbreak began in February, around 300,000 people have died from malaria, while tuberculosis has likely claimed over 600,000 lives".
Even Lassa fever, which shares many of the symptoms of Ebola, kills many more than Ebola and frequently finds its way to the US.
Many scientists worry that the seasonal flu virus (which itself contributes to 500,000 deaths) could mutate into something much more deadly. Indeed, the 1918 flu pandemic ended up killing between 3% and 5% of the world's population.
A brief history of the disease
Ebola was discovered in 1976 by scientists in Antwerp who were analysing the blood sample of a nun who had died from a mysterious disease while working in Zaire (now the Democratic Republic of the Congo). The disease spread to nearby villages, killing 280 people, before strict quarantine policies stopped it. Since then there have been several very brief, small outbreaks.
The disease occurs in bats and some local animals. Left untreated, Ebola has a mortality rate of up to 90%, though care can improve the chances of survival to around 50%.
Experts believe that the reason that this year's outbreak has been so extensive is that it occurred in more densely populated areas, close to the borders of several countries so it has been difficult to stop people circulating.
