The answer is: A teacher in a school smells gasoline and begins to feel nauseous; soon most of the schools population is also feeling nauseous. Later, it is discovered that there was never a gas leak.
Social contagion refer to a social phenomenon when the behavior that displayed by a certain individual create a chain of reaction that make other people around him/her start to display the same behavior . (such as when the students mimic the nauseous behavior after they observe their teacher did it)
Social contagion tend to have a stronger effect if the person who initiate the contagious behavior is in the position of authority/importance (such the teachers in school environment or Actors in film set)