Answer:
Flu shots reduce the likelihood of others catching the flu.
Explanation:
Any transaction has benefits & costs to some members of society.
Eg: An individual's purchase of flu shots from a doctor - patient benefits from treatment for which its pays the monetary cost to the doctor, which is latter's income benefit.
However, these both are patient's & doctor's private benefit & costs.
Externalities may cause extra harm or benefit to other uninvolved parties, without any monetary exchange for that harm or benefit. Beneficial are positive externalities, harming are negative externalities.
In this case, treatment is the positive externality : It has extra benefit for other people who are less probable to catch the flu, without having paid for that prevention (eg vaccination) or any other way.
