In the mid-20th century, poliovirus paralysed half a million children a year, in rich countries as well as poor. In 1952 there were 57,628 cases in the United States. Following the development of vaccines by Jonas Salk and Albert Sabin, polio declined markedly in North America and Europe. The US had its last case in 1979, the UK in 1982. There were still, however, about 350,000 cases a year in the mid-1980s, predominantly in countries where the state did not have the money or capacity to implement mass vaccination programmes. The Global Polio Eradication Initiative was formed in 1988 by the WHO and national governments to finance and organise immunisation campaigns. It precipitated a sharp reduction in polio: there were 37 cases in the world in 2016, a fall of 99.9 per cent. But the disease stubbornly persists in Nigeria, Afghanistan and Pakistan.

Eliminating an infectious disease by deliberately eradicating the causative agent has happened only twice: smallpox fell in 1977 and rinderpest – cattle plague – in 2011. Polio should be next, but the murders in the last week of vaccinators in Pakistan – one in Charsadda, two in Peshawar, five in Karachi – have stopped the eradication programme. Nothing new for the Taliban, who blocked polio immunisation in Waziristan earlier this year. Things haven’t been helped by the disclosure that Dr Shakil Afridi, whose activities helped to locate Osama bin Laden in Abottabad, operated under the cover of a bogus immunisation programme run by the CIA.