Mass Psychogenic Illness (MPI, or Mass Hysteria) is when a group of people start to think they have been harmed.
Explanation:
To elaborate and to give an example, if a group of people believe the government poisoned their water, they will start acting as though they have been poisoned. But this is all placebo, in their mind.
Mass Hysteria is mainly spread and started through rumors, and perpetuated by paranoia on the people's part. But physically they are fine, mentally they experience a lot of anxiety and stress.
