The answer to that is that those millions of doses of vaccine annually jabbed into the arms of children are a needle-thin line protecting humanity from disease. Their greatest triumph has been the elimination of smallpox, which once killed millions and scarred the survivors. If you've ever wondered why milkmaids were so often shown in landscape paintings, it is because their faces were unscarred - as Edward Jenner surmised, their tendency to catch the milder cowpox protected them from smallpox. Jenner developed his cowpox vaccine over 200 years ago, but the disease wasn't conquered until the massive vaccination campaigns of the 20th century.

Smallpox has not been the only conquest. Anyone who has travelled in Africa or India has been shocked by child beggars dragging polio-ravaged limbs. Do you remember pictures of wards of people gazing out of rows of iron lungs? Those patients weren't in hospital for treatment; they were trapped in those machines for life after polio paralysed their respiratory systems. Polio has been nearly defeated. The World Health Organisation's heroic efforts are now tackling the last redoubts of the disease, delivering the heat-sensitive vaccine by camel in Sudan, bicycle in India and boat in Cambodia and Vietnam.

True, all infectious diseases were not eliminated, even in the West, in the 20th century. Meningitis, caused by the Hib and MenC bacteria, annually killed hundreds of children in the UK into the 1990s. In 1992 the Hib vaccine was introduced and since then Hib disease has almost disappeared. The UK was the first country to introduce the MenC vaccine in 1999; there was a big reduction of MenC disease in 2000.

There are downsides to all health interventions, including vaccines. Vaccination may have kept you free of measles in childhood, but if you are unlucky enough to contract measles as an adult, it is likely to be more severe. We should remember why measles, mumps or rubella are diseases of childhood in places where they are still common: because infectious agents generally infect children soon after birth. If those children survive, they became immune. For most infections, disease is the best vaccine. But that immunity comes at a huge cost, as children that don't quickly develop immunity will die.

In the UK, vaccinating children is like voting. A single vote, or a single vaccination, is rarely going make any difference. If everyone else's children are vaccinated, herd immunity will ensure that deadly bacteria or viruses cannot circulate. A few people can then afford to be "conscientious objectors", because the vaccination of the majority will keep disease at bay. But as with voting, if everyone stops, the system collapses. Measles, mumps, rubella or diphtheria haven't gone away. They are still among us, ready to break out if we let our defences down. A recent outbreak of measles in the Netherlands was triggered by low rates of vaccination: 2,961 children were infected and most recovered. But three died. If too many of us refuse to have our children vaccinated, then parents will soon be discussing epidemics that are far more deadly than nits.

If vaccine-preventable diseases take hold again in the UK, the well-fed children of Guardian readers will not suffer the worst. Measles has a 10% mortality rate among in the malnourished. But pointing to social deprivation as the cause of disease doesn't save lives. Vaccines do. And they are one of the cheapest ways to do it.

· Johnjoe McFadden is professor of molecular genetics at the University of Surrey.