Pokérus

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

A player being told their Pokémon has caught Pokérus

In the games

Unlike other status ailments, Pokérus 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 the Pokérus, it gains double the effort values from battling (i.e., fighting a Magikarp will give two Speed 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 about two days. However, the Pokérus timer can be delayed by several methods including placing the infected Pokémon in a PC Box.

Pokérus has a 3 in 65,536 chance of being generated on a Pokémon, either wild or bred. 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 Shiny Pokémon), and the fact that most would not even recognize a Pokémon with the virus and may flee from it, many players would never have a Pokémon with the virus.

In later generations, the Pokérus became more accessible, 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 beginning Generation IV. In fact, it is common courtesy to infect Pokémon to be traded on the Global Trade Station with Pokérus within some parts of the fandom. Also, a Pokémon's chances of being infected with the virus very slightly increase when a Pokémon has been traded from another game.

Infection

When infected, a Pokémon's status screen will indicate it much as it does any other status condition (with icon being different for Generation III and IV, while Generation V has the same as Generation IV and Gen II having no icon at all). 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 may 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.

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 V, 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, Day Care, Stadium 2 (in Generation II), Pokémon Box Ruby & Sapphire (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

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

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, <player>?
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!"

Technical information

In Generation II, Pokérus is stored in a Pokémon's data structure as a single byte at offset 0x1C. Within the byte, the upper 4 bits (upper nibble) and lower 4 bits (lower nibble) are represented in hexadecimal format as XY, where X represents the specific strand of Pokérus the Pokémon has contracted, and Y represents the number of days remaining before the infected Pokémon is cured of the virus.

This means that there exist 256 possible states of Pokérus: 1 unaffected state, 240 infected states (39 legitimate and 201 hacked), 15 cured states:

• An unaffected state is always represented as 0. (hex 00)
• An infected state is represented by any number between 1 and 255 that has a remainder when divided by 16.
  • For example, 33 (hex 21) represents an infected state, since 33 divided by 16 would produce 1 as a remainder.
• A cured state is represented by any number between 1 and 255 that does not have a remainder when divided by 16.
  • Possible 'cured' values can be 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224 or 240. (hex values 10, 20, 30, 40, 50, 60, 70, 80, 90, A0, B0, C0, D0, E0 or F0 respectively)

Remaining days

Whenever the game's internal clock strikes midnight, every Pokémon in the player's party has their Pokérus value decreased by one point. Once the Y value reaches 0, the Pokémon will be cured of Pokérus.
For example, if a given Pokémon's Pokérus value was 65 (hex 41), it would decrease down to 64 (hex 40) and therefore be cured. However, if the value was 82 (hex 52), it would instead decrease down to 81. (hex 51) This means that if consistently left in the player's party, it is possible for a Pokémon to remain infected with Pokérus for a potential maximum of 15 days.

Specific strain

Whenever the game assigns a Pokérus value to any given Pokémon, Y values between 1 through 4 are used, depending upon the X value generated. (However, any Y value can be attained through hacking, meaning that if left in the player's party, a Pokémon can only remain infected with Pokérus for a legitimate maximum time span of 4 days.)
The value of X represents the specific Pokérus strain that the Pokémon has been infected with:

Pokérus strand # (X value)	Duration of infection (Y value)	Pokérus value
0	1 day	1 (hex 01)
1	2 days	18 (hex 12)
2	3 days	35 (hex 23)
3	4 days	52 (hex 34)
4	1 day	65 (hex 41)
5	2 days	82 (hex 52)
6	3 days	99 (hex 63)
7	4 days	116 (hex 74)
8	1 day	129 (hex 81)
9	2 days	146 (hex 92)
10 (hex A)	3 days	163 (hex A3)
11 (hex B)	4 days	180 (hex B4)
12 (hex C)	1 day	193 (hex C1)
13 (hex D)	2 days	210 (hex D2)
14 (hex E)	3 days	227 (hex E3)
15 (hex F)	4 days	244 (hex F4)

For example, if a Pokémon has a Pokérus value of 50, (hex 32) meaning it has strain 3 and will be cured in 2 more days. If it were to infect another Pokémon, the new Pokémon would obtain the Pokérus value of 52, (hex 34) meaning it would instead be cured in 4 more days. The strain remains constant when copied over. The remaining number of days prior to being cured however, depends on the strain itself.

Currently, there is no known way to determine how specific strain are generated by the game. (Other than the fact that there is a 3 in 65,536 chance of a wild Pokémon carrying Pokérus) However one of these 16 strain is illegitimate: Strain 0. This is because when Pokérus values between 1 through 15 (hexadecimals 01 through 0F) eventually decrease down to 0 (hexadecimal 00), a Pokémon would not only be cured of Pokérus, but would also lose any sign of having been infected in the first place - meaning that they could become reinfected at a later point in time.

As a result, out of the 240 possible states of Pokérus infection, only 39 of them are considered legitimate. This can be calculated by adding up all of the 15 legitimate strains and multiplying each of them by the possible number of remaining infected days. (2 + 3 + 4 + 1 + 2 + 3 + 4 + 1 + 2 + 3 + 4 + 1 + 2 + 3 + 4 = 39)

Erasing Pokérus from a Pokémon

If a Pokémon with any stage of Pokérus (active or cured) is traded back to a Generation I game, or withdrawn from Stadium 2 by a Generation I game, all traces of that Pokémon having had 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 Pokérus multiple times by trading it back and forth, once it becomes cured of a given infection.

In Generation III

The Pokérus operates in much the same way as in Generation II, though it can infect 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, and a Pokémon with Pokérus will keep it indefinitely (it can't spread Pokérus, either) , until it is transferred to the Hoenn-based games. The Pokérus icon is .

Nurse's comments

"Your Pokémon may be infected with 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."

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.

It will also double the effects of the new EV increasing items as well.

In HeartGold and SoulSilver, Professor Elm calls the player a little while after healing their first Pokémon with 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. The Pokérus icon is .

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 comments

This section is incomplete. Please feel free to edit this section to add missing information and complete it. Reason: Needs Elm's Quotes on Pokérus

In Generation V

The mechanics remain more or less the same, sharing similarity to the strain phenomenon in Generation II. However, a difference would be that the icon that would appear if a Pokémon has previously had Pokérus would be a pinkish color instead of the former yellow/orange color. The Pokérus icon is still .

To determine if any Pokémon in the player's party is to be infected, the game calls the Mersenne Twister table and then discards the lower half (16 bits) of the 32 bit number, then does & 0x3FFF (bitwise and) on the result. If the result is 0, a Pokémon in the party will 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 that party member already has been infected, nothing else will happen.

Now that the given Pokémon is to be infected, the game takes the next value from the Mersenne Twister table, discards the lower 24 bits, then enters a loop to determine the strain. The strains are biased towards the lower number strains. Once a strain has been determined, the duration the infection will last is determined. For a given strain s, the duration is s+s%4.

Refer to the Generation II table for strains and duration, as it is exactly the same. However, strains 0x0 and 0x8 are not obtainable legitimately.

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, 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. 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.

In the manga

In the Pokémon Adventures manga

Pokérus was discussed in Different Dimension Battle IX, when Platinum's team was discovered to have this virus and, as a result, became stronger. Her Froslass, Pachirisu, and Cherrim 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 Yanase Berlitz realise the infection is, in fact, Pokérus.

Trivia

• Pokérus is an example of a mutualistic virus (while seemingly oxymoronic, a "virus" is classified by infecting cells, rather than negative effects), in which both host and virus benefit. Viruses with possible positive effects exist in the real world , but all of Pokérus's known effects are objectively positive. A notable difference between biological viruses and the Pokérus is that biological viruses infect a host cell by mutating the cell's DNA. Pokérus only affects the host Pokémon's effort values, not the host's individual values, which are regarded as the Pokémon analogue to DNA.
• Using a trend called the Magnemite Chain/Coil (where trainers use the GTS to trade Magnemite in order to increase visitors to their Join Avenue in Black 2 and White 2) a new trend is starting where trainers are using the Magnemite chain to spread the Pokérus to other trainers.

References

This game mechanic article is part of Project Games, a Bulbapedia project that aims to write comprehensive articles on the Pokémon games.