With a population of more than 170 million, Nigeria is Africa's most populous nation and there were fears that Ebola would take hold when a Liberian-American arrived with the disease in July. An initial contact list of 281 people soon increased to a staggering 894 - each of them visited and checked repeatedly for signs of infection. But the sleuthing did not end there. Specialists then calculated how many people were living within a particular radius of the 894 people who were being monitored. This depended on the density of the housing in each particular area. The result was that...