THERE IS NO cure for polio. The virus, which is spread from person to person, attacks the brain and spinal cord, bending children’s limbs into horrific spider-like contortions. It has killed or crippled millions, rich and poor alike (President Franklin D. Roosevelt famously contracted the disease). Luckily, there is good news. Today marks three years since Nigeria, Africa’s most populous country, reported a case of the virus. As recently as 2012, the country accounted for more than half of all global polio cases, according to the World Health Organisation (WHO). It is now on the verge of wiping out the disease. By 2020 the entire African continent may be declared polio-free.

This milestone was a long time coming. Polio was once endemic in more than 125 countries, where it paralysed 350,000 children every year. But in 1988 the WHO launched an effort to eradicate the disease by vaccinating children en masse. The virus was first eliminated from the rich world: America was declared polio-free in 1979 and Europe in 2002. The developing world has proved more difficult. Over the decades tens of thousands of health-care workers have trekked through dangerous and remote areas of the world vaccinating almost 3bn children. The result is that, in the past 30 years, the incidence of polio has dropped by more than 99%. In 2018 there were just 33 cases of the disease.