The agent stimulates the body's immune system to recognise the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognise and destroy any of these micro-organisms that it later encounters.
Vaccines may be prophylactic (example: to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic.
The term vaccine derives from Edward Jenner's 1796 use of cow pox (Latin name - Variola vaccinia, adapted from the Latin vaccīn-us, from vacca meaning cow), to inoculate humans, providing them protection against smallpox.
Vaccination protects hundreds of millions of animals worldwide from disease and possibly death.
Animals, just like humans, suffer from a range of infectious diseases. As veterinary medicine has advanced, prevention of disease has become a priority as healthy food comes from healthy animals. One of the best means of preventing disease is by creating immunity in the animal - this is usually achieved by vaccination.
The principle of vaccination has been established for over 200 years. Since those early days, enormous strides have been made in the development of vaccines, which have helped to prevent - and in some cases eliminate - many diseases in humans, farm animals and family pets.
Animals which develop disease often require treatment with medicines, so vaccination helps reduce the amount of pharmaceuticals used in the treatment of animals. Vaccination presents no hazard to consumers of produce from vaccinated animals.
Not all animals need every vaccine. Some, like clostridial disease prevention, are basically routine, just like childhood vaccination programmes. The vaccination programme chosen for farm animals depends on the management system, the location of the farm and the history of the herd or flock (and whether or not a disease is likely to be encountered). Most farm animals are young, and these animals (just like children) are often more susceptible to infection. So, for example, calves often need to be protected by vaccination against respiratory disease.