After entering the
Hall of Fame, every time the player wins a battle, there is a ~0.5% (328/65536) chance that the game will generate an outbreak; this does not apply to link battles, e-Reader battles, the Safari Zone, or the Battle Tower/Frontier. After an outbreak is generated, the television program announcing it will appear the next day. Once the television program has been watched, the outbreak becomes active.
[1] Once outbreak has been activated, it will last until the end of the next day - during this time there's a 50% chance the game will generate the swarming Pokémon instead of using the usual encounter table (effectively halving the encounter rate for non-swarming Pokémon). However, seemingly due to an oversight, once a particular savefile generates an outbreak, it will never generate another one outside of
record mixing.
[2]
The
record mixing feature of
Pokémon Ruby, Sapphire, and
Emerald allows players to give an outbreak they've seen to other players, regardless of whether that outbreak would
naturally occur in the player's game. The traded outbreaks will be announced on television in the following days after mixing records.