Vaccination was discovered by chance at the end of the 18th century when a country doctor, Edward Jenner, learnt that milkmaids did not fall victim to smallpox. At that time smallpox accounted for around 10 per cent of deaths in childhood and early adult life, while the pustular rash left some survivors with hideous facial scars. The milkmaids who escaped smallpox had all been infected with cowpox, a trivial short-lived illness causing a few blisters on the hands. Jenner's stroke of genius was to try an experiment that was, to modern eyes, horrifyingly unethical: he deliberately infected a small boy with fluid from cowpox blisters and then brought him into contact with infectious victims of smallpox. The boy was immune. Within a few years, Jenner's vaccination with cowpox was being used throughout Europe; vaccination has now made smallpox extinct.
Jenner and his contemporaries had no real understanding of the nature of infectious diseases, and not until Pasteur and Koch established the germ theory was any further advance possible. Pasteur developed a vaccine against rabies, but further vaccines came only with better understanding of the body's natural immune defences. For hundreds of years folk-healers had known that children who survived smallpox, measles, and other childhood fevers rarely if ever had a second attack of the same illness. Jenner's vaccination was based on a serendipitous finding that the virus of cowpox produced, in effect, a very mild attack of smallpox from which everyone would recover. Many modern vaccines are made by altering a dangerous virus to make it cause a mild version of a natural illness which nevertheless induces the body's immune defences to respond in the same way as to the full-blown illness.
All vaccines based on living viruses carry a slight risk that the illness they cause may be more severe than predicted. Very occasionally, vaccination against smallpox has killed someone; polio vaccine causes a paralysing attack of poliomyelitis in around one child in one million. Advances in molecular biology are leading to safer vaccines, however, as scientists are able to determine the precise chemical structure of a disease-causing virus and then select a fragment of the structure as the basis for a vaccine. As more vaccines become available, health economists have to calculate the benefits of protecting children against minor illnesses such as chicken pox.
No effective vaccines are yet available against malaria or Aids, but the research is in progress and by early in the next century infectious illnesses may well have become very rare causes of death in the first half of the lifespan.Reuse content