Pokérus

The Pokérus (Japanese: ポケルス Pokérus), from "Pokémon virus", is a microscopic life-form that may attach to, first appearing in the Generation II. It is a beneficial mechanic that a Pokémon can obtain.

In the games
When a Pokémon has the Pokérus, it gains double the effort values from battling (e.g., fighting a will give two  EVs, rather than one). Effort points gained from vitamins and wings are not doubled. The infected Pokémon can infect other Pokémon with the virus for a period of one to four days. However, the Pokérus timer can be delayed by several methods such as by placing the infected Pokémon in a PC Box.

While it is represented similarly to status conditions, the Pokérus is not a status condition, so it cannot be healed at a Pokémon Center or with any status condition healing item.

Pokérus has a 3 in 65,536 or approximately a 1 in 21,845 chance of being generated on one of the undefined's Pokémon after a battle, making it rarer than encountering or hatching a Shiny Pokémon. If any of the player's Pokémon are infected, the Pokérus may spread among Pokémon in the player's party after any battle.

From Generation IV onward, the Pokérus became more accessible, with worldwide trading facilitated by Nintendo Wi-Fi Connection.

Infection
When a Pokémon is infected with the Pokérus, their status screen will display an icon indicating this special "status" the same way status conditions would be noted. If the Pokémon becomes affected by a status condition such as, the Pokérus icon will be temporarily replaced until the status condition is cured. As long as the Pokémon is infected with the Pokérus, it can spread the virus to other Pokémon in the player's party.

The Pokérus may spread if an infected Pokémon is in the player's party after a battle. The Pokérus may only spread to a Pokémon directly adjacent to an already infected Pokémon and only if they have never had Pokérus before. From Generation III onward, s may catch the Pokérus like any other Pokémon.

Becoming cured
A Pokérus infection only progresses towards cured status when a new day starts with the infected Pokémon in the party (or if, when the game is loaded, it is not the same day as it was when the game was saved). The number of days before a Pokémon will be cured of the Pokérus can vary from one to four days. Once this time has passed, the Pokémon becomes cured and will be immune to the virus in the future.

Due to the beneficial nature of the Pokérus, players will often place an infected Pokémon in the PC where it will keep the infection indefinitely, so that it may be withdrawn to spread the virus at will. Other options include putting a Pokémon in the or sending it to  in Generation II,  in Generation III, My Pokémon Ranch in Generation IV, or Pokémon Bank in Generation VI.

In Generation III, if the player deposits an infected Pokémon inside a PC, saves inside a Pokémon Center prior to midnight, shuts off the game, resumes the game after midnight, withdraws the infected Pokémon from the PC, and exits the Pokémon Center, it is possible for the infected Pokémon to be cured of the Pokérus upon stepping outside of the building.

Technical information
The Pokérus is stored in a Pokémon's data structure as a single byte. In hexadecimal, this can be represented as a two-digit number XY. The upper 4 bits of the byte, X, represent the specific strain of the Pokérus the Pokémon has contracted. The lower 4 bits, Y, represent the number of days remaining before the infected Pokémon is cured of the virus.

A Pokémon is or has been infected if X (the strain) is a nonzero value. If Y (days remaining) is nonzero when X is also nonzero, this means the Pokémon is currently infected. If Y is 0 and X is nonzero, then the Pokémon is "cured" of the Pokérus. If both X and Y are 0, then the Pokémon has never been affected by the Pokérus.

Whenever the game's internal clock strikes midnight, every currently infected Pokémon in the player's party has their Pokérus value decreased by one. Once the Y value reaches 0, the Pokémon will be cured of the Pokérus.

Strains
Whenever the game creates the Pokérus on a Pokémon, the value assigned to Y (days) depends on the value assigned to X (strain). Specifically, the number of days will be set to X modulo 4 + 1. In other words, the higher two bits of X are irrelevant to the "strain".

Whenever the Pokérus spreads from an infected Pokémon to a new Pokémon, the new Pokémon inherits the infected Pokémon's strain of the Pokérus (X) and Y is set to the default value for that strain. As an example, if a Pokémon has the Pokérus with an X value of 7 and a Y value of 2 (2 days remaining before it will be cured) and it infects another Pokémon, the new Pokémon will get the Pokérus with an X value of 7 and a Y value of 4 (4 days before it will be cured).

X values of 0 are not legitimate, since upon being "cured" the Pokémon would appear to have never had the Pokérus at all.

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 and onward, it will be a small face. Though the virus is gone, the Pokémon will still gain twice the s it would have gained before the virus.

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 "tiny 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. A Pokémon with active Pokérus will list its status as such, and a Pokémon that has had the Pokérus and is cured will have a small dot near its HP meter on the status screen.

If a Pokémon with any stage of the Pokérus (active or cured) is traded back to a Generation I game, or withdrawn from Pokémon 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, thereby making it possible for a Pokémon to legitimately contract the Pokérus multiple times by trading it back and forth, once it becomes cured of a given infection. The only benefit of doing so would be to allow other Pokémon to contract the virus.

Nurse's comments
"Your Pokémon appear to have tiny life forms stuck to them. Your Pokémon are healthy and seem to be fine. But we can't tell you anything more at a Pokémon Center."

Elm's explanation
"Hello, ? I discovered an odd thing. Apparently there's something called Pokérus that infects Pokémon. Yes, it's like a virus, so it's called Pokérus. It multiplies fast and infects other Pokémon too. But that's all. It doesn't seem to do anything, and it goes away over time. I guess it's nothing to worry about. Bye!"

In Generation III
The Pokérus operates in much the same way as in Generation II, though it can infect s too. Also, it can only be contracted or cured in. In, as well as and , since the time function is absent, a Pokémon with the Pokérus will keep it indefinitely (it can't spread the Pokérus, either), until it is transferred to the Hoenn-based games.

Nurse's comments
"Your Pokémon may be infected with Pokérus</sc>. Little is known about the Pokérus</sc> except that they are microscopic life-forms that attach to Pokémon</sc>. While infected, Pokémon</sc> are said to grow exceptionally well."

In Generation IV
The mechanics remain the same from Generation III. Additionally, much like storage in and in the PC, storage in My Pokémon Ranch will keep a Pokémon's Pokérus status indefinitely. Starting in this Generation, the Pokérus will double the EVs obtained from the EV-enhancing items.

In, Professor Elm calls the player a little while after healing their first Pokémon with the Pokérus and tells them that it has no effect and will wear off. If he is called back, he will claim that Pokémon will level up better with the Pokérus while infected.

Nurse's comments
"Your Pokémon may be infected with the Pokérus. Little is known about the Pokérus except that they are microscopic life-forms that attach to Pokémon. While infected, Pokémon are said to grow exceptionally well."

Elm's explanation
When calling the player: "Hello, ? I discovered an odd thing. Apparently there's something called Pokérus</sc> that infects Pokémon</sc>. Yes, it's like a virus, so it's called Pokérus</sc>. It multiplies fast and infects other Pokémon</sc> too. But that's all. It doesn't seem to do anything, and it goes away over time. I guess it's nothing to worry about. Bye!"

When called by the player: "Hello, ? It seems that Pokémon</sc> that have been infected with Pokérus</sc> level up better. We're not quite sure why..."

In Generation V
The mechanics remain more or less the same, but now the icon that appears if a Pokémon has previously had the Pokérus is pinkish in color instead of the former yellow/orange color.

Pseudorandom number generation
To determine if any Pokémon in the player's party is to be infected, the game calls the table to get a 32-bit random number, discards the lowest 16 bits, and then discards the highest two bits (a bitwise and with 0x3FFF). If this number is 0, the game will choose a Pokémon in the party to be infected.

To determine what party member is to be infected, the game takes another value from the Mersenne twister table, multiplies it by the party count, then discards the lowest 32 bits of the result. If this number is an Egg, the calculation is repeated. Otherwise, if that party member has already been infected, nothing else will happen.

If a Pokémon will be infected, the game takes the next value from the Mersenne twister table and discards the lowest 24 bits (if the lowest three bits of this result are all 0, another number is chosen). If any of the highest four bits are nonzero, the highest five bits are discarded. The resulting number will be X, the Pokérus strain; the duration of the virus will be set to X modulo 4 + 1.

These calculations mean that the strains 0 and 8 will never occur, and that the remaining strains are biased towards the lower numbers (with 1 through 7 being much more common than 9 through 15).

Nurse's comments
"Oh... It looks like your Pokémon may be infected with the Pokérus. Little is known about the Pokérus, except that it is a microscopic life-form that attaches to Pokémon. While infected, Pokémon are said to grow exceptionally well."

In Generation VI
In Generation VI, the Pokérus does not affect EVs gained from Super Training. However, it increases the rate of Double-Up Bags that are received during the training regimen. The icon that appears if a Pokémon has the Pokérus has been changed to spell out the full word, while the cured icon uses a design similar to the Gen V icon, with the mouth and eyes having a white color.

Nurse's comments
"Oh... It looks like your Pokémon may be infected with the Pokérus. Little is known about the Pokérus, except that it is a microscopic life-form that attaches to Pokémon. While infected, Pokémon are said to grow exceptionally well."

In Generation VII
During battle, a Pokémon's summary will display a "Pokérus" icon beside its name if it has the Pokérus. The icon remains after the Pokémon is cured. An Egg infected with the Pokérus does not have this symbol on its summary screen. It can still spread the virus normally, and the Pokémon that hatches from the Egg will immediately acquire the icon.

Nurse's comments
"Oh... It looks like your Pokémon may be infected with the Pokérus. Little is known about the Pokérus, except that it is a microscopic life-form that attaches to Pokémon. While infected, Pokémon are said to grow exceptionally well."

In the anime
In Oaknapped!, Dr. Namba explained to that the mysterious viral lifeform Pokérus is an important component of his Pokémon Power Acceleration Project (PPAP), a secret project that aims to infect Pokémon with the Pokérus to evolve them at a faster rate. However, all of his specimens had died out. Professor Oak also explains that the term "Pokérus" was coined by combining the words "Pokémon" and "virus" together. Later, Professor Oak began researching about the Pokérus and Team Rocket's true intentions of using it.

In the Pokémon Adventures manga
The Pokérus was discussed in Alternate Dimension Showdown IX, when 's team was discovered to have this virus and, as a result, became stronger. Her, , and had it prior to being taken from the hospital, and then it spread to the rest of her party. At the hospital, Daisy Oak and realize that the infection is, in fact, the Pokérus.

Trivia

 * The Pokérus is an example of a, in which both host and virus benefit.

In other languages
Pokérus Pokérus Pokérus Pokérus ポケルス 寶可病毒