Immunisation protects people against harmful infections before they come into contact with them. If enough people in the community are immunised, the infection can no longer spread from person to person and the disease can die out altogether. This is how smallpox was eliminated from the world, and polio has been removed from many countries. Despite excellent hospital care, significant illness and death still occur from diseases which can be prevented by immunisation. Either term is used now to describe prevention of a disease by using a vaccine.
Immunisation is the preferred term.
Immunisation gives a very good level of protection against many important diseases. However, sometimes, even when all doses of a vaccine have been given, not everyone is protected against the disease. This can often be years later. Boosters are required because immunity to the disease can diminish with time leaving the person once again vulnerable to infection. There are quite a number of infectious diseases which can be prevented by immunisation.