A crowd is a group of people, also known (especially in the United States) as a mob. The crowd may have a common purpose or set of emotions, such as at a political rally, at a sports game, or during looting, or simply be made up of many people going about their business in a busy area (eg shopping).
The fight-or-flight response, also called the acute stress response, was first described by Walter Cannon in 1929. His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing. This response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms.