When an Outbreak occurs, 1 space is filled on the Outbreaks Track. Then, 1 Disease cube is placed on every city connected to the city of the outbreak. If any of the cities already has 3 cubes of the disease color, a 4th cube is not placed on those cities. Instead, in each of them, a chain reaction outbreak occurs after the current outbreak is resolved.
When a chain reaction outbreak takes place, 1 space is filled on the Outbreaks Track. Then, cubes are placed as above, except cubes are not added to cities that have already had an outbreak (or a chain reaction outbreak) as part of resolving the current Infection card.
After an outbreak, a city may have disease cubes of multiple colors on it; up to 3 cubes of each color.
If the Outbreaks Track is completely filled, the game ends and your team has lost!