Pandemic: On the Brink includes new event cards, new role cards, rules for five players, and optional game challenges to increase the difficulty of the Pandemic base game. These new challenges, which can be used individually or combined for even more difficult play, are as follows:

The Virulent Strain challenge makes one disease become particularly deadly in unpredictable ways.
The Mutation challenge adds a fifth (purple) disease that behaves differently than the original four.
The Bio-Terrorist challenge pits one player against the others!