As in the original Pandemic, each player takes on a specific role to limit the spread of four viruses across the globe and research a cure. But then things … change. As you play more games in the season, the viruses mutate, rules change, cities rise and fall, and new character options and abilities (and penalties) come into play. Each session is different from the one before because game modifications are permanent and carry over between sessions. The continuous gameplay creates the feeling of a coherent, evolving story, and we were always curious (and terrified) to find out what would happen next.
Source: Tim Barribeau (New York Times)