Student: If a person has an immunity to infection by a microorganism, then that microorganism does not cause them to develop harmful symptoms
Since many people are exposed to staphylococcus without developing any harmful symptoms, it follows that they have an immunity to infection by this microorganism
Everything morally right is just, but some actions that best serve the interests of everyone are not just. Thus, some morally right actions do not serve the interests of everyone.