Paradoxically, it is the very virulence of Ebola – it rapidly incapacitates its victims and kills between 50 and 70 per cent of them – that has meant all outbreaks predating this one have rapidly burnt themselves out. A virus wants its host to stay alive and mobile for as long as possible, so it can infect other people. With Ebola, once the symptoms start, you will not be going anywhere (one of the reasons that screening air passengers arriving in the UK may be pointless). Ebola is also hard to catch – it requires direct contact with bodily fluids, and you cannot infect others until you are symptomatic.