Across the continent, Ebola barely rates among big killers, with far more deaths attributable to malaria or even diarrhoea. But the exponential spread of infections means that, unchecked, it could quickly grow extremely large. The World Health Organisation (WHO) last month said it hoped to control the outbreak within nine months and at 20,000 cases, but other experts think it may take 12 to 18 months—and infect hundreds of thousands of people—before it is brought under control.