Receiving medical services creates positive externalities, especially when the treatment shortens the time that a person can get another person sick. People who did not receive the treatment get a benefit because they have a lower probability of getting sick.

For example, people who do not get the flu shot have a reduced chance of getting the flu for every person that does receive the flu shot. This is an example of a positive externality because they receive the lower probability of illness despite not taking part in the transaction.