From the post:
The most striking feature of Legacy games is their permanence. Unlike many other tabletop games, the decisions you make in a Legacy game are felt in future games. New objectives are introduced, new components are added, and some are even destroyed. This permanence is the heart of the Legacy format and adds a thrilling new layer to your campaigns.
In Pandemic Legacy: Season 1, this permanence takes many forms. Cities slowly descend into chaos as panic spreads, diseases mutate, and new cards come into play. Over the course of your campaign, the characters you play can forge relationships, developing lasting friendships or getting locked in bitter rivalries. Far from being simple story beats, these elements tie back into the gameplay, becoming a part of the fabric of your world.