So what has made this outbreak so big? The overriding factor could be urbanisation. In the past, village outbreaks remained small, unless people went to hospitals. "Population size and high mobility make it hard to do contact tracing," says Peter Walsh at the University of Cambridge. Cities provide more chances to spread the virus, something that may also have enabled the spread of HIV. According to the African Development Bank, the continent has had the world's highest urban growth rate for 20 years, and the proportion of Africans living in cities will rise from 36 per cent to 60 per cent by 2050.