In 1976 a blue thermos flask and a letter from a doctor in Kinshasa arrived at a lab in Antwerp, Belgium.
The flask contained blood samples from a Belgian nun who had recently fallen ill from a mysterious sickness.
When the scientists peered down an electron microscope they realised they were looking at a virus unlike anything they had seen before.
The only way to stop it spreading was to go to the centre of the outbreak, as the BBC's Science editor David Shukman explains.
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