From Bulbapedia, the community-driven Pokémon encyclopedia.
Mass outbreaks (Japanese: 大量発生 mass outbreak), also known as swarms, Pokémon outbreaks, or wild outbreaks, are a feature introduced in Generation II that allows the player to catch Pokémon that are usually rare or otherwise unobtainable. They appear from Generations II to V.
In the games
In Generation II, if the player registered a certain Trainer in the Pokégear, that Trainer occasionally calls to tell the player that they saw or heard about a large number of Pokémon in a certain area. Until midnight (in-game time), the swarming Pokémon appear in the given area with much greater frequency than the usual 1%. Only one swarm can appear per day. The player can force a particular swarm to appear if a swarm has not already appeared that day by first deleting all possible phone numbers except the swarm Trainer's and then repeatedly toggling Daylight Savings Time by speaking with the player's Mom in New Bark Town. There is a 50% chance of triggering a phone call every time DST is changed.
In Generation III, in addition to making rare Pokémon temporarily more common, this feature can be combined with record mixing to allow the player to catch Pokémon that cannot be caught at all normally. Pokémon encountered through mass outbreaks will also have a predefined set of moves.
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. Outbreaks last until the end of the day after they became active, during which time the swarming Pokémon will have an appearance rate of 50%.
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.
Mass outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex and has entered the Hall of Fame, a "swarm of Pokémon" will appear every day in a particular location, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events. Information about mass outbreaks can be found in Sandgem Town by talking to Lucas/Dawn's sister or by watching the Sinnoh television DPPt, or by listening to Professor Oak's Pokémon Talk HGSS.
When players all join the same group, they will all experience the same outbreak on any given day.
The species of an outbreak seems to be determined using a formula, because it is not chosen randomly when the player starts the game that day. Even if the date is edited, the next swarm will always be the same. It is also possible that the species is determined the day before, though there is no additional evidence to support this. If the DS clock is changed or a different DS is used, then the game will detect this and will not change the species swarming until 12:00 AM.
Many of the Pokémon obtained through mass outbreaks are outside of the Sinnoh Pokédex, and are usually otherwise exclusive to other regions.
Due to the high encounter chances for Pokémon that appear during outbreaks, they make for longer and better chains when using the Poké Radar.
The feature continues to appear in HeartGold and SoulSilver, where the wild Pokémon available include all those from Generation II, plus Ditto, Chansey, and Pokémon from Hoenn and Sinnoh. Information on swarms can now be found by listening to Professor Oak's Pokémon Talk on the Pokégear radio.
The gates in Unova have a monitor inside that will alert the player to where an outbreak is taking place.
Unlike in previous games, mass outbreaks can be manipulated by soft resetting the game, but only if the player has not saved their game at all that day. When the game is restarted, there will be a different Pokémon swarming.
During winter, the monitor may occasionally not report on outbreaks, which is due to parts of Route 8, where Croagunk can be found, becoming frozen during Winter and not allowing the Pokémon to be caught there. All outbreaks in these games have a level range of 15-55.
All outbreaks in these games have a level range of 40-55 and an encounter rate of 40%.
In the anime
A mass outbreak of Pikachu in the anime
In Ivysaur's Mysterious Tower!, Vermilion City was the site of a mass outbreak of Bulbasaur and Ivysaur. Ash and Goh learned that the former grassland, now the site of a new Gym, was the ideal site for the Bulbasaur line to absorb the solar energy and wind needed to undergo evolution.
In Gotta Catch a What?!, Ash and Goh ventured into the mountains to discover a reported Pikachu outbreak. They learned that Pikachu had gathered because the area was rich in Thunder Stones, helping the Pikachu, including Goh's newly caught one, evolve into Raichu en masse.
In JN051, Goh learned of a mass outbreak of Geodude at the Rock Tunnel. After arriving there with Ash, he almost immediately found one and caught it.
In other languages