Initially, not all characters or scenarios may be available. To unlock all eight playable characters, you typically need to complete the 'Outbreak' scenario with each of the starting four characters. New scenarios are unlocked sequentially by completing the previous one. For example, completing 'Outbreak' unlocks 'Below Freezing Point'. To unlock the final scenario, 'Decisions, Decisions', you must complete all preceding scenarios. Some bonus scenarios or difficulties might require completing the game on harder settings or achieving specific ranks.
