I remember summers just after the war when polio outbreaks led to panic measures such as the closing of public swimming pools. Not much was known about this illness, popularly known as 'infantile paralysis'. In common with many other virus infections, before vaccines were developed virtually everyone caught the wild disease at some time in childhood. Most hardly noticed it - just another sore throat. A few, however, became ill with a severe headache and stiffness in the neck, and in some symptoms progressed to weakness of the muscles of the legs and trunk, sometimes the whole body.
The virus attacks the nerve cells in the spinal cord that power the muscles, leaving sensation and consciousness unaffected. In those years around 600,000 children worldwide became paralysed each year. A few were totally paralysed and were kept alive only by treatment in a primitive respirator known as an iron lung. Fortunately, half of the children who developed paralysis made a full recovery; many others had only slight permanent weakness. Nevertheless, thousands were left with a weak leg, the muscles of which wilted away with time; an affected leg would become shortened, and the child - and then the adult - limped for ever.
Parents, well aware of the unpredictability and untreatable nature of polio, did all they could to protect their children: swimming pools were closed partly because this was thought to reduce the risk of transmission, but also because of evidence that children most likely to become paralysed were those who had been physically very active in the days before they became ill.
All these fears were swept away by the introduction of the Salk and Sabin vaccines in 1955 and 1961. The oral Sabin vaccine used in Britain and most of the world is a living virus which mimics natural infection and very rarely - perhaps once in five million doses - causes a paralytic illness. WHO believes that worldwide around 83 per cent of children are immunised against polio; but around 15,000 cases of paralytic polio are still reported each year, and the true figure must be higher.
What are the messages? First, vaccination works: it defeated smallpox and is on the way to defeating measles and polio. These were diseases which, in the words of an African proverb, children had to survive before their mothers could count them. It may be difficult to persuade parents to vaccinate children when they have never seen a friend or relative suffering from polio. Secondly, for the time being at least travel to Africa and Asia will mean exposure to a risk of polio; people not fully vaccinated are at risk. Meanwhile, the WHO's immunisation programme needs our continuing financial support.