By Philip Whiteside, international news reporter
Ebola is a virus that causes a serious illness which often leads to death if it is not treated.
It was first identified in 1976 in the Democratic Republic of the Congo in a community next to the Ebola river.
During the outbreak, there were 318 cases identified, but 88% of those affected died. The death rate makes it one of the most deadly diseases in history – far worse than the estimated 30-60% of Europeans who died of bubonic plague in medieval times.
In 1967, as a result of a possibly related outbreak to DR Congo, the disease killed 151 people in what is now South Sudan.
Since then, there have been at least 25 outbreaks – the most famous of which killed nearly 11,000 people in Guinea, Sierra Leone and Liberia between 2014 and 2016.
A few outbreaks have occurred in the West but the vast majority have hit countries across a large area of sub-Saharan Africa.
All have peaked and then subsided and many have occurred years apart, and often hundreds or thousands of miles apart.
It is only recently that scientists have figured out that the movement of Ebola across vast distances and its tendency to disappear and reappear is down to its natural home in the animal kingdom.
Professor Daniel Bausch, the director of UK Public Health’s Rapid Support Team for Ebola and a senior figure in organising the UK’s response to the outbreak in West Africa, told Sky News the reason the disease can travel so far.
“We think that it’s probably maintained in nature in bats,” he said.
“When Ebola started near the small town of Gueckedou in west Africa in 2013, there was evidence that the first person infected, a small child, was playing around a tree where bats roosted.
“So you can imagine that it was either direct infection from the faeces or for example the bat may have been chewing on a mango and that mango falls and the boy chews on the same fruit, and it’s passed on through the saliva.
“Bats are the reservoir for various other sister viruses to Ebola. So that’s the predominant theory.
“So, humans can get infected in numerous ways. They can get infected through unwitting casual contact, through bat faeces, saliva or food.
“Also, although it seems like a culturally odd thing to many of us, bats are a food source in many areas of the world – people do catch bats and cook them.
“It’s probably not people cooking the bats and eating them as the virus would be inactivated through the cooking, so it’s the person who would be catching and butchering the animal who would be most at risk.
“Then there’s a third way. Rather than the bat chewing on the mango and a human chewing the mango, it could be a monkey, or something like that, that gets infected through that process and then the human goes and hunts that animal (like a monkey) with the Ebola virus and the person who butchers that animal gets infected.”
He said bats have been known to travel huge distances, but also are very social animals so the potential for Ebola to spread from bat to bat is very high.
“Bats can have large migratory patterns,” he said.
“It remains speculation how the strain of Ebola got to Guinea. But it’s not that one bat or even a colony of bats needs to migrate all in one go to West Africa.
“It may be that the process that eventually resulted in bat being infected in Guinea, started a decade ago – a colony of bats went from one cave in central Africa to a cave a little bit more to the northwest and as time went on passed the virus on to other colonies, eventually resulting, perhaps years later, in bats being infected in Guinea.
“Bats are flying mammals. Some can travel large distances.
“It is possible that it can go anywhere the bats can go but it may also be that viruses are very specifically paired to specific species. We think they evolved together over thousands of years. There are no fruit bats in the US or UK so you wouldn’t have introduction from bats in those countries.”
He said scientists had found evidence of at least three species of bat carrying Ebola, but it was not known which one passed the disease to humans.
But the gaps in outbreaks can be explained by the likelihood that Ebola-carrying bats will move around and will only occasionally come into the kind of contact with people that leads to cross-infection.
“There have been 10 outbreaks in the DRC,” Prof Bausch said. “From those, we can tell they each represent distinct events where there was introduction from the wild.
“If you want to use the term ‘it’s lying dormant’, it’s lying dormant in the bat population. It’s not that it’s lying dormant in the human and then coming out.”
Despite Ebola’s wide range and ability to come back, Prof Bausch said he was confident it would not evolve into a form any more dangerous than it is at present.
He said: “This is an infectious disease but if you compare it with other diseases it’s actually not dangerous in terms of catching it. People often confuse how transmissible a virus is with how lethal it might be.
“For example measles – if someone in a room has measles, you are much more likely to catch it if you are in the same room or aeroplane, because it can be airborne.
“With Ebola, you have to have direct contact and it has to be in the later stages of the disease. If you are standing a few metres away from someone with Ebola, even if they have a very severe case, if you have no contact with their blood or bodily fluids, then your risk is essentially zero.
“There may be other species of the Ebola virus out there but if it were to have incredible mutations those mutations maybe so different that they would not be able to replicate in animals.
“Some degree of mutation you could see, but I don’t think there’s the risk of the sort of thing you sometimes see in the movies that the Zaire virus is suddenly going to change to become airborne, or maintained by some other animal, that’s something you see in Hollywood.”