The dispatch of troops to west Africa may seem an odd priority when American forces are preparing to confront jihadists in Iraq and Syria and are stretched thin elsewhere. Ebola is a disease that is usually absent from human populations, has been quickly stamped out in the past and in its worst recorded outbreak has thus far caused 3,000 known deaths (see article). Moreover it is unlikely to spread widely in rich countries with good health-care systems. Set against killers such as HIV, the virus that kills some 1.6m people a year, or tuberculosis (TB), which takes another 1.3m lives, an expensive fight against Ebola may seem a misallocation of resources.
Yet Ebola is now growing exponentially, with the number of new cases roughly doubling every three weeks or so. In Monrovia, the capital of Liberia, it is thought to be doubling every two weeks. Previous outbreaks were usually in rural villages where it was easier to contain. At this rate of progress, small numbers quickly become big ones, and there is a real risk of the disease spreading to cities such as Lagos, which is home to more than 10m people. The longer Ebola is allowed to replicate in humans, the greater the risk that it will become more contagious. Some virologists fret that it might even acquire the ability to be transmitted through the air by coughs and sneezes. Although this seems unlikely, nobody wants to find out just how quickly Ebola can adapt to humans.