Pokérus

The Pokérus is a special status condition that a Pokémon may get, first appearing in the Generation II games. Unlike other status conditions, it cannot be healed at a Pokémon Center or with any status ailment healing item, and can only be obtained by the Pokémon in question once. When a Pokémon has had the Pokérus, it gains double the effort points (i.e., fighting a Magikarp will give two Speed EVs, rather than one). The virus takes about two days to wear off of an infected Pokémon, but the period of time can vary.

When it was introduced, the Pokérus had a 3 in 65,536 chance to be on a wild Pokémon. Through battling or capturing this wild Pokémon, players could get this helpful virus onto their own Pokémon, and subsequently spread it around. Due to the overwhelmingly low chance of encountering the virus (about 1/3 of the chance to encounter a Template:Shiny2 Pokémon), however, most players would never have a Pokémon with the virus.

In later generations, the Pokérus became more likely to get, with several event Pokémon during Generation III having it when transferred into a player's game, and with worldwide trading facilitated by the Nintendo DS's Wi-Fi capabilities during Generation IV, it is essentially ubiquitous. In fact, it is common courtesy to infect Pokémon to be traded on the GTS with Pokérus within some parts of the fandom.

Infection

When infected, a Pokémon's status screen will indicate it much as it does any other status condition, as ^P_Krs. When another normal status condition, such as Sleep, affects the Pokémon, the Pokérus icon will be replaced. As long as the Pokémon is infected with Pokérus, it can spread the virus to the other Pokémon in the player's party. The infection will spread if the infected Pokémon is in the main party after a battle. An uninfected Pokémon must be next to a Pokémon infected with Pokérus in the main party for the virus to spread. If Pokémon that have already been infected and cured from Pokérus are in the main party and are on either side of a Pokémon currently with the disease, Pokérus will not spread after any amount of battles. Pokémon that are put into the PC will keep the status indefinitely.

In Generation II

When the player first encounters the virus in one of the Generation II games, the nurse at the Pokémon Center will make note of it when the Pokémon is first healed after contracting it, saying that there are "small life forms" on the Pokémon. After leaving the Pokémon Center, Professor Elm will call the player to tell them that the virus has no effect and will wear off.

In Generation III

The Pokérus operates in much the same way as in Generation II, though it can infect Pokémon eggs too. Also, it can only be contracted or cured in Ruby, Sapphire, and Emerald. In FireRed and LeafGreen, as well as Colosseum and Pokémon XD, since the time function is absent, a Pokémon with Pokérus will keep it indefinitely, until it is transferred to the Hoenn-based games.

In Generation IV

The mechanics remain the same from Generation III. Additionally, much like storage in Pokémon Box Ruby & Sapphire and in the PC, storage in My Pokémon Ranch will keep a Pokémon's Pokérus status indefinitely.

Nurse’s comments

“Your Pokémon may be infected with Pokérus.
Little is know about the Pokérus except that they are microscopic lifeforms that attach to Pokémon.
While infected, Pokémon are said to grow exceptionally well.”

After having the virus

Once a Pokémon's immune system has fought off the virus, they cannot spread it further, nor can it be spread to them by other Pokémon. In Generations II and III, a dot will appear on the Pokémon's status screen to indicate that they have had the virus previously, while in Generation IV, it will be a small smiley face. Though the virus is gone, its positive effects will remain, most specifically the ability for that Pokémon to gain double EVs.

Any Pokémon on the player's party at the moment the system clock reaches midnight after a certain number of days in that position (number of days depends on the strain and generation) will become immune. An easy way to keep a "vessel" for future spreading is to always send the infected Pokémon to the PC, Daycare, Stadium 2 (in Generation II), Box (in Generation III), or My Pokémon Ranch (in Generation IV) before midnight. The game also recognizes immunizing if the system is turned off and on again on the next day.

In Generation II, if a Pokémon with any stage of Pokérus, active or cured, is traded back to Generation I or withdrawn from Stadium 2 by a Generation I game, all traces of that Pokémon having had the Pokérus will disappear. This is because a Pokémon's Pokérus status is not saved in any form in the Generation I games.

Despite being called a virus, the Pokérus' relationship to its host is more Symbiotic as it helps its host instead of harming it.

In the anime

In Oaknapped, Professor Namba explained to Professor Oak that the mysterious viral lifeform Pokérus is an important component of the Pokémon Power Acceleration Project (PPAP), a secret project that aims to infect Pokémon with Pokérus to evolve them at a faster rate. However, all of his specimens have died out. Later, Professor Oak began researching about the Pokérus.