In some circumstances, firefighters use fire to fight fire by creating an intense explosion very close to an uncontrollable blaze that they wish to extinguish, thus momentarily depriving it of the oxygen it needs to continue burning. 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. In some cases, a business will close down some of its operations, its intention being to position the company to be more profitable later even though this involves expenses in the current period. Some parents read their children fairy tales containing allegorical treatments of treachery and cruelty, with the intention of making them less emotionally vulnerable to these phenomena when they encounter them later in life.
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.
Which one of the following best illustrates the principle that the passage illustrates?