infectious
adj.adj. spreading quickly from one person to another. This can describe a disease or a positive feeling like laughter that other people start to feel too.
adj. capable of being transmitted by infection, typically via a pathogen; by extension, describing a mood or behavior that spreads rapidly through a group.
Her laughter is infectious and makes everyone in the room smile.
The doctor warned that the flu is highly infectious during the first few days of symptoms.
Despite the grim economic forecast, the CEO maintained an infectious optimism that eventually permeated the entire management team and revitalized the project.
Borrowed from New Latin īnfectiōsus. By surface analysis, infect + -ious.
Commonly used both in medical contexts and figuratively to describe emotions or social behaviors.