Vaccines are dead or deactivated disease-causing organisms, or mild derivatives of such organisms. They stimulate the body's immune system to recognize, destroy and "remember" these organisms so that when the more deadly or stronger organisms attack, the immune system has already built up its defenses.

Maintaining high immunization rates is important to ward off outbreaks or epidemics of an infectious disease.

Some countries adopt compulsory vaccination against certain diseases, while some other countries leave it to parents to make the decisions on whether or not to inoculate their children.