Vaccines contain either a greatly weakened form of the bacterium or virus that causes a disease, or a small part of it. When the body detects the contents of the vaccine its immune system will be primed to make the antibodies (substances that fight off infection and disease) required to fight off the infection.
If your child comes into contact with the infection, the immune system will recognise it and be ready to protect him or her by producing the right antibodies. Because vaccines have been used so successfully in the UK, diseases such as diphtheria have almost disappeared from this country.
Take a look at the history and development of immunisation (external link) across the globe.
Years of extensive laboratory and clinical tests are undertaken during a vaccine's development.
Before a vaccine is put into general use it has to be licensed. In order to be granted a licence, the manufacturers have to demonstrate its quality, safety and efficacy in preventing the particular disease for which it is intended.
Before clinical trials can be started the relevant regulatory authorities and Ethics Review Committees (external link) have to give their approval.
The Medicines and Healthcare products Regulatory Agency (MHRA) (external link) monitors the safety of vaccines through the Yellow Card Scheme (external link).
The information collected through the Yellow Card Scheme is constantly reviewed by independent experts and used to inform national immunisation policy decisions.