Herd Immunity

Herd immunity is the protection a whole population gains when enough people are immune that a disease can no longer spread easily. It shields babies and the vulnerable, and it helped the world eradicate smallpox through mass and ring vaccination.

The model applies beyond medicine. Some protections only work collectively, like team security habits, family rules about scams, or simply staying home when sick. Your small action becomes part of a barrier you will never see. The threshold varies and protection can fade, so it needs renewing.