When doctors vaccinate a patient, their intention is to expose him or her to a weakened form of a disease-causing pathogen and thus to make the patient better able to resist the pathogen and less likely to develop a severe form of that disease later.
Some police departments energetically pursue those who commit minor crimes; in doing so they intend to provide examples to deter people who might be tempted to commit more-serious crimes best illustrates the principle that the passage illustrates.