Allusions of a threat spread through a group causing panic, each person's fear reinforcing that of the next and in a vicious circle.

Examples:
The people of Maple Street gradually transform into an hysterical mob when they thought an alien monster was masquerading amongst them in "The Twilight Zone" (1959) story "The Monsters Are Due on Maple Street" (tz1959e1x22).

References:
https://en.wikipedia.org/wiki/Mass_hysteria