Every year at the beginning of flu season, many people, including the elderly, get a flu shot to reduce their chances of contracting the flu. One result is that people who do not get a flu shot are less likely to contract the flu. Getting a flu shot results in a externality.

negative externality            

Explanation:

In simple words, negative externality refers to the loss that an unrelated third party experiences due to any economic transaction that occurs between the other two independent entities.

Under this concept the two parties do not deliberately effect the third party and generally that third party do not get any chance to tackle the loss before it actually happens. Diseases happening to general public due to pollution by factories is the prime example of negative externality.

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