Malaria in Africa has historically been a rural problem, because the mosquitoes that carry it mostly breed in rural water. Anopheles stephensi breaks that rule. It thrives in the water tanks, cisterns, and construction containers of cities, and since arriving in Djibouti in 2012, it has been spreading across a rapidly urbanizing continent.
A genomic study published in Science has reconstructed how it got there, and the answer involves geography as much as biology.
