1. something that is contagious
his/her enthusiasm is catching
This expression is used to refer to something that can be easily passed on or given to somebody else. That can be something literally infectious, like a cold or flu; or something figuratively infectious, like a feeling or emotion that spreads quickly or has a strong influence, particularly among a group of people. The former context is more common socially, the latter professionally. A similar term is 'spread like wildfire.'