The term contagion has roots in the Latin word contagio, which translates “with touch.” So contagion literally refers to transmission of symptoms by personal contact.

Definitions of social contagion vary, but often include the idea that thoughts, feelings, and other psychological states can spread through a community or group by suggestion, gossip, or imitation. Social scientists have documented these patterns of transmission, but researchers disagree on the underlying mechanisms of social contagion.

In the “War of the Worlds” radio broadcast of 1938, Orson Welles caused a nationwide panic with his realistic dramatization of an alien invasion of Earth, which “reported” that Martians had landed and annihilated a force of 7,000 soldiers.

An estimated 1 million radio listeners believed that real Martians had invaded, and panic broke out across the country. Frightened motorists clogged highways in an effort to escape the aliens.

Social scientists have documented how human beings’ tendency toward conformity helps spread such fears and behaviors.

Studies have focused on both emotional contagion (the spread of mood through populations) and behavioral contagion (the spread of behaviors through populations).

Research has documented how social contagion can spread hysteria symptoms, self-harm behavior, aggression, rule violation, and financial panic.

For example, when the stock market fluctuates we often see a spread of financial fears. An uptick in unemployment or threat of rising interest rates can push investors into a selling panic, which then fuels further fear and more selling.

Another form of social contagion stems from the fear of violence, including terrorism. This form can cause us to misinterpret the environment around us.

For example, the 2001 anthrax attacks in the United States led to fears of opening mail. People would jump at the sight of loose sweetener granules or leftover talc because they thought it could contain lethal bacteria.

Some airlines even banned artificial sweeteners for a time.

Unusual activity on any form of public transportation or in a large crowd can trigger strong emotional reactions and sometimes an exaggerated response.

Of course, such reactions can serve the public interest — as we’ve seen with several terrorist attacks recently thwarted by vigilant and heroic observers — but it can also lead to undue stress.