Ebola isn't a particularly contagious disease.
For every person that has the disease in an outbreak, two people catch it from them. Compare that with measles, where one person usually infects another 18.
So why is the disease so deadly?
First, although it requires direct contact with someone showing the symptoms of ebola, even a very low dose is enough to infect a person.
And when that happens, the virus is particularly ferocious.
Ian Jones, professor of virology at the University of Reading, told Sky News: "When the virus infects an individual, it begins its replication cycle - all viruses want to make more of themselves. But it has a mechanism to hide under the immune system's radar.
"When the immune system eventually kicks in, it does so in an over-reactive fashion and that damages host tissue, as well as trying to kill the virus."
What are the mechanisms behind that?
When ebola enters the body, it targets dendritic cells in the immune system.
Normally, when a virus is detected, these cells tell other cells to produce antibodies.
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Gallery: The Desperate Fight To Contain The Ebola Outbreak
A man rests outside the clinic.
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A woman is comforted after medical officials remove her husband, who is suspected of having the disease.
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Officials try to prevent themselves from spreading the disease.
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A local who has just brought his brother to the centre. He had to rely on plastic bags tied around his hands to try to protect himself.
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A man thought to be infected with ebola waits for treatment.
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Patients wait to be seen by medical staff.
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Workers try to decontaminate themselves.
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A worker with a child who may have caught ebola.
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A makeshift hand-washing station in Monrovia.
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Decontaminated boots of medical staff.
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The basic conditions make containing the disease very difficult.
Ebola prevents that signal getting out. As far as the immune system knows, everything inside the body is fine.
Left alone, ebola then begins replicating rapidly. It then spreads into the bloodstream, infecting the whole body. Cells start to break up and die, in huge numbers. That finally triggers the immune system, which kicks in - far too aggressively.
Ordinarily when you get sick, the body releases proteins called cytokines. Some of these cells tell your blood vessels to become more permeable. This is to let antibodies travel through the body more quickly to fight the disease.
But once ebola has taken hold of your body, the immune system reacts much too aggressively - and launches a cytokine storm.
This causes blood vessels to become far too permeable, and they leak. At the same time, the body's blood clotting mechanisms also act abnormally.
This causes internal and external bleeding and is why ebola is known as a haemorrhagic fever. It causes tissue damage and organ failure.
In effect, the body turns on itself, then liquefies. Eventually, organs fail, and there is a catastrophic drop in blood pressure, causing death.
Once this process has begun, there is little that can be done to moderate it. This is why available new therapies, like ZMapp, have a far higher success rate the earlier they are given.
But the worst ever outbreak of the disease will also give us the most information as to exactly how it works.
Around half of those who develop Ebola do survive. No one is quite sure why, but they do seem to then be resistant to the disease afterwards. By studying the antibodies developed biologically, we may find paths to new treatments.
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