Pokérus

The Pokérus is a rare virus featured primarily in the main Pokémon series of video games. It may occur naturally and affect a Pokémon. The Pokérus first appears in Pokémon Gold and Silver Versions. The Pokérus is very transmissible, meaning that it can quickly be spread between different Pokémon. The name Pokérus is a portmanteau of Pokémon and "virus," and despite the name, the Pokérus bestows a beneficial effect to the Pokémon who has it and those who were cured of it.

Pokémon series
In the Pokémon series, the Pokérus accelerates the statistical development of its host Pokémon, making it gain effort values at twice the speed. This effect is cumulative with the Macho Brace, and an infected Pokémon with such an item effectively trains at four times faster than normal. The Pokérus is mainly contracted from a Pokémon battle, although there is only a 2 in 65535 chance of contracting a Pokérus in this manner. If a Pokémon on the active party has the Pokérus, there is a high chance (50% in most games) that another Pokémon in the party contracts the Pokérus in every round of battle. The active Pokémon can also contract the Pokérus in this manner if the first member has the Pokérus. It can take a few days for a Pokémon to be cured of Pokérus. The effect of Pokérus remains even after a Pokémon is cured but no longer becomes contagious. Any Pokémon who has been cured from Pokérus becomes immune to it. If an infected Pokémon is in the Pokémon Storage System, they cannot be cured of Pokérus.

Pokémon Gold and Silver Versions / Pokémon Crystal Version
In Pokémon Gold and Silver Versions and Pokémon Crystal Version, a Pokémon with Pokérus in the Pokémon Storage System can spread it to other Pokémon. In the status screen, a Pokémon cured of Pokérus is marked by a small, black dot above the HP bar.

Because the Pokérus is not present in Pokémon Red and Blue Versions and Pokémon Yellow Version: Special Pikachu Edition, trading a Pokémon from Pokémon Gold, Silver, or Crystal Version to either of the Generation I games does not retain the information of whether the Pokémon has the Pokérus or if it has been cured from it. Therefore, trading a cured Pokémon to one of the Generation I games and then back into Pokémon Gold, Silver, or Crystal Version resets the possibility of it contracting Pokérus.

Pokémon Ruby and Sapphire Versions
In Pokémon Ruby and Sapphire Versions, the chance of another Pokémon contracting the Pokérus is reduced to 33%. Since these games, any Pokémon who has Pokérus is marked by a purple rectangle that is abbreviated to "PKRS" in the Summary window. Starting with Pokémon X and Y, the status icon is fully spelled out as Pokérus.

Pokémon FireRed and LeafGreen Versions
In Pokémon FireRed and LeafGreen Versions, because there is no time-based system, a Pokémon with Pokérus has it permanently unless it is traded to either Pokémon Ruby or Sapphire Version.

Pokémon Emerald Version
In Pokémon Emerald Version, the Pokérus retains its functionality from Pokémon Ruby and Sapphire Versions.

Pokémon Colosseum / Pokémon XD: Gale of Darkness
In Pokémon Colosseum and Pokémon XD: Gale of Darkness, the Pokérus can only be contracted from Pokémon that can be snagged. Like Pokémon FireRed and LeafGreen Versions, any Pokémon with Pokérus has it permanently due to the lack of a time-based system.

Other spin-offs
There are several spinoff games, such as Pokémon Stadium 2 or Pokémon Box Ruby & Sapphire, that allow for a Pokémon, including its Pokérus status, to be traded over. Similar to Pokémon FireRed and LeafGreen Versions, Pokémon Colosseum, and Pokémon XD: Gale of Darkness, a Pokémon with the Pokérus status in these games cannot be cured of it.