The Economist explains: How disease forecasts can go wrong

The Ebola epidemic that took off last year in three west African countries—Guinea, Liberia and Sierra Leone —is finally fading. The last three weeks have each seen around 120 new cases, the lowest level since July 2014. In total, around 22,000 people are thought to have been infected, and 9,000 to have died. But predictions last year were much higher, including a worst-case scenario by America’s Centres for Disease Control and Prevention (CDC) of 1.4m cases, reported and unreported, by January 20th. How are such predictions made, and how can they go wrong? Disease predictions come from mathematical models that group the population at risk of a disease into categories and describe how people move between them. Both the categories and the flows depend on the disease. With Ebola, there is a long lag between infection and the first symptoms, during which it is not contagious. And recovered sufferers are immune....