ERADICATING a disease is the sort of aim that rich countries come up with, and poor ones struggle to reach. But for some diseases, the pattern is now reversed. These are the ailments for which vaccinations exist. The trend is most obvious for measles, which is so contagious that at least 95% of people must be vaccinated to stop its spread (a threshold known as “herd immunity”). Many poor countries are well above that. Eritrea, Rwanda and Sri Lanka manage to vaccinate almost all children. By contrast several rich countries, including America, Britain, France and Italy, are below herd immunity.

Cases of measles in Western countries plummeted as vaccination rates increased after the 1980s, when the jab became routine. But in recent years outbreaks have returned in countries where vaccination has faltered, for reasons that our article explains. Last year Europe missed the target it set itself in 2010 to eliminate measles, with outbreaks in several countries and nearly 4,000 cases. America and Germany have both recently seen their first measles death in more than a decade.