Pandemic is one of the favorite games of everyone I play with. One of the strengths of Pandemic is that it is a cooperative game. All the players are working together to cure the diseases that are represented with small cubes covering the board, which is the only way to win the game. There are, however, numerous ways to lose the game: have eight outbreaks, run out of time (run out of cards in the player deck), and running out of disease cubes to place on the board.
The original Pandemic has really good replay-ability on its own with multiple difficulty settings. I have the first expansion: Pandemic On the Brink. It adds new event card and player roles as well as new challenges. Virulent Strain selects one of the disease colors and gives it specific abilities with each new epidemic which makes it harder to treat and cure. Mutated Strain adds a fifth disease that behaves differently from the other four. Bio-terrorist introduces a player role that works against the other players in the game.
My current favorite set-up is to play with both the Virulent Strain and Mutated Strain challenges on an easier difficulty.
I haven't yet gotten a chance to play with the Bio-Terrorist challenge. I hope to change that soon since one of the members of my game group is eager to be the Bio-Terrorist.
You can find Pandemic and Pandemic on The Brink on Amazon or at a Local Game Store