MALARIA has been a scourge for most of history. In recent years, a good deal of progress has been made against the disease. But, as the World Malaria Report 2017, published on November 29th by the World Health Organisation, explains, that progress seems to be tailing off. The reason is unclear. Fingers are, however, being pointed at a decline in a technique known as indoor residual spraying (IRS). This involves coating the interior walls of buildings in malaria-prone areas with insecticide, to kill mosquitoes that land on them. The report says that the proportion of people at risk of malaria who are protected by IRS has fallen from 5.8% in 2010 to 2.9% in 2016. Again, it is unclear why. It may be an unintended consequence of the sensible policy of rotating, over the years, the insecticides used for IRS. This helps suppress the evolution of insecticide-resistance in mosquitoes. But it often means replacing conventional pyrethroid insecticides with more expensive alternatives, which some people cannot afford.