The word Immune is derived from the Latin meaning exempt. In the late 18th century a physician named Edward Jenner noticed that milk-maids, although frequently developed cowpox, very rarely developed smallpox. He carried out an experiment in which he injected pus from a cowpox lesion into a healthy boy. Obviously the boy developed cowpox. Jenner then injected the same boy with puss from a smallpox lesion. But the boy didn't develop smallpox. This is because the cowpox virus is very similar to the smallpox virus only it's much less harmful. As the two viruses were so similar, if the immune system could overcome the less dangerous cowpox it would be able to quickly recognise and destroy the much more threatening smallpox virus.
The body had developed cells which "remembered" cowpox-like viruses. This is how vaccination came about. Vaccination is derived from the word vacca meaning cow. Later on Louis Pasteur showed that attentuated (weakened) live viruses and heat killed bacteria can be used for vaccination against disease.
Types of Immunisation
Active. Active immunisation is termed vaccination and elicits a response from the body that provides protective immunity.
Passive. Passive Immunisation is the injection of antibodies into the body to provide short-term protection.
Vaccination is intended to provide long-term protection after its administration. So the most important quality a vaccine must have is to generate immunological memory.
Currently Available Vaccines
A very important point to note is that live vaccines should never be given to immunosuppressed individuals.