Contagion
Etymology
From Latin contagio, "a touching together," from con-, "with," and tangere, "to touch" (the root of "tangible" and "contact"). The literal sense is transmission by touch. Long the word for how infection passes between people, it now applies just as readily to emotional contagion in a crowd or financial contagion spreading between markets.
noun · /kun-TAY-jun/
The spread of disease by contact, and by extension the rapid spread of anything catching, from panic to ideas.
What it names
A word that captures how quickly things jump from person to person — germs, moods, rumors, and market fear all "spread."
Example
Yawning is mild social contagion — one person starts and the whole table follows.
Related idea
Immunity is resistance to contagion; epidemiology studies the paths contagion takes.