They form part of India’s army of 2.5 million frontline health workers – the human machinery behind the world’s largest immunisation campaign. Shameema estimates she has administered polio drops to more than 3,000 children during her years as a vaccinator. Tanzeela has given polio drops to about 800 children since she joined the programme. The oral polio vaccine uses a live but weakened form of the virus developed by a Polish scientist, Albert Sabin, in the late 1950s. Up to 27 million children under five will be targeted in one day during a polio vaccination drive in India.