A vaccine, or immunisation, is a biological preparation that contains a fragment of a particular disease, or a synthetic substitute. It is injected into the bloodstream to help the body develop antibodies to, and immunity from, various diseases.
Because a small amount is injected, the individual will not experience the full disease (although mild side effects may occur). However, because the body has been exposed to a small element of it, antibodies will still form.
Antibodies fight infection and, once antibodies against certain diseases have developed (for example, measles) the individual is very unlikely to catch the disease again, even if directly exposed to it.
Vaccines are an excellent way of protecting children - and adults - from potentially devastating conditions such as measles, rubella and polio. Thanks to vaccines, small pox has now been eradicated.
From the age of three months, every child in the UK will be offered a range of immunisations which are administered at various points throughout their childhood.
These vaccines include measles, mumps and rubella (MMR); whooping cough; tetanus; meningitis c and diphtheria. Adolescent girls will also be offered a vaccine against the human papillomavirus, which is known to cause cervical cancer.
While all parents are offered vaccines for their children, it is a personal choice and can be declined. However, the vast majority of parents will follow the immunisation programme.
Some children will be unable to receive certain vaccines, but only if they have experienced serious illness such as cancer, or present a risk of severe reaction.