How do vaccines work?
When the body is exposed to foreign organisms, such as bacteria and viruses, the immune system produces antibodies against them. Antibodies help the body recognise and kill the foreign organisms. They then remain in the body to help protect the body against future infections with the same organism. This is known as active immunity.
The immune system produces different antibodies for each foreign organism it encounters. This establishes a pool of antibodies that helps protect the body from various different diseases.
Vaccines contain extracts or inactivated forms of bacteria or viruses that cause disease. These altered forms of the organisms stimulate the immune system to produce antibodies against them, but don't actually cause disease themselves.
The antibodies produced remain in the body so that if the organism is encountered naturally, the immune system can recognise it and attack it, thus preventing it from causing disease.
Each bacteria or virus stimulates the immune system to produce a specific type of antibody, and this means that different vaccines are needed to prevent different diseases.