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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. 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 Magikarp will give two Speed EVs, rather than ☀️ one). Effort points gained from Vitamins and Feathers 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 player'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 Sleep, 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. Eggs 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. The Pokémon still gains double effort values when ☀️ cured.
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 Day Care or sending it to Stadium 2 in Generation II, Pokémon Box Ruby & ☀️ Sapphire in Generation III, My Pokémon Ranch in Generation IV, Pokémon Bank in Generation VI and Generation VII, or Pokémon ☀️ HOME in Generation VIII.
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. This does not happen in any other generation.
In Generation VIII, the game will check on the ☀️ next available random event increment after midnight. This can result in a Pokémon that was infected while in a box, ☀️ and withdrawn directly after resuming the game from sleep, being subsequently cured of the Pokérus after the next battle or ☀️ similar event.
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".
Strain X values Duration
(Y value) A ☀️ 0, 4, 8, 12 1 day B 1, 5, 9, 13 2 days C 2, 6, 10, 14 3 days ☀️ D 3, 7, 11, 15 4 days
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 copied directly from the infected Pokémon. 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 be infected with ☀️ Pokérus with an X value of 7 and a Y value of 2. Infecting other Pokémon does not reset the ☀️ Y value to the default value for a given strain on the newly infected Pokémon, nor for the original host.
In ☀️ Pokémon Emerald onward, X values of 0 and 8 do not generate legitimately, as the RNG doesn't permit these values ☀️ to generate. Additionally with a value of 0, 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 EVs it would ☀️ have gained before the virus.
Differences between generations
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. At the end of a battle, the virus has a ☀️ 1/3 chance to spread. If the infected Pokémon is adjacent to two Pokémon who have never had it before, one ☀️ of them will catch the Pokérus. Pokérus can infect Eggs as well. An indicator for Pokérus will not show up ☀️ on the Egg's status screen, but once it hatches, it will appear under its status on the status screen like ☀️ normal.
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.
The Pokérus cannot randomly occur before the player has visited Goldenrod City[1] (but it may spread between ☀️ Pokémon before then). If a Pokémon in the party has the Pokérus, other Pokémon cannot randomly contract it; they can ☀️ only receive it from that Pokémon.[2]
Infection and spread details
This section is incomplete.
Please feel free to edit this section to add ☀️ missing information and complete it.
Reason: Description of the random byte generation algorithm, which is that used by other random byte-needing ☀️ routines
The party is first iterated over to determine if any member has the Pokérus. If so, the spread code is ☀️ called for each member with the Pokérus and not the new infection code, and if not vice versa.
The spread code ☀️ checks if a random byte is less than 85 and if the party has multiple members before proceeding as follows: ☀️ if the spreader is the last party member, or if a random byte is less than 128, iterate backwards, otherwise ☀️ iterate forwards. If the member being iterated over has an active infection, they are skipped over and become the spreader. ☀️ If they are clear and never had the Pokérus, they are infected. The Y value for an infection is generated ☀️ from the X value as it is for a spontaneous infection, unlike in future games where it is copied. The ☀️ iteration stops when a member is newly infected, when it reaches a cured member, or when it would be about ☀️ to pass through the beginning or end of the party.
The new infection code first checks if the flag for having ☀️ visited Goldenrod City is set, terminating if not. Then, it checks if a random byte is equal to zero and ☀️ another random byte is less than three, terminating if not. It then chooses a random party member by generating a ☀️ random byte and keeping its bottom three bits (bitwise and with seven), rerolling until that number is a valid party ☀️ index. If that party member has ever had the Pokérus, the code terminates. Otherwise, a random byte is rolled, rerolling ☀️ if zero until not zero. This byte shall be represented as AB (not to be confused with registers A and ☀️ B) in the same way that the Pokérus byte is represented as XY. If A is not zero, B's bottom ☀️ three bits (bitwise and with seven) are copied as a four-bit value, incremented, and put into X, and X's bottom ☀️ two bits (bitwise and with three) are copied as a three-bit value, incremented, and put into Y. Otherwise, it is ☀️ likely intended to copy B into X, but, perhaps due to a misplaced load, zero is copied into X instead; ☀️ Y is then derived from X with the same code as with A not zero, but since X is always ☀️ zero, Y is always one. (If the misplaced load is placed in a more logical place, the Y value ends ☀️ up always being one anyway, so there may have been more required effort to make the code work as intended.)
X ☀️ values of zero and eight can occur naturally: the latter is as designed, the former is due to the above ☀️ bug. Due to this bug, an X value above eight cannot occur naturally. Probabilities of each strain is as follows:
X ☀️ values Chance to occur 0 15/255
(5.88235%) 1, 2, 3, 4, 5, 6, 7, 8 30/255
(11.76471%) 9, 10, 11, 12, 13, ☀️ 14, 15 0/255
(Cannot naturally occur)
Generation III
The Pokérus 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 the Pokérus ☀️ will keep it indefinitely (it can't spread the Pokérus, either), until it is transferred to the Hoenn-based games. When a ☀️ Pokémon can spread the Pokérus, it has a 1/3 chance to spread it to both adjacent Pokémon.
Otherwise, the Pokérus operates ☀️ in much the same way as in Generation II, but it is now possible for the Pokérus to be randomly ☀️ contracted even when another Pokémon in the party already has it (and the Pokérus can spread after the same battle ☀️ where it was contracted)[3], and there is no location requirement before the Pokérus can randomly be contracted. Eggs infected with ☀️ Pokérus will now indicate as such.
In Ruby and Sapphire only[4], X values of 0 and 8 do occur naturally, whereas ☀️ in Pokémon Emerald onward, the RNG does not allow them to occur at all. The following is the chance to ☀️ obtain each strain in Ruby and Sapphire:
X values Chance to occur 0 30/255
(11.76470%) 1, 2, 3, 4, 5, 6, 7 ☀️ 31/255
(12.15686%) 8, 9, 10, 11, 12, 13, 14, 15 1/255
(0.39215%)
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. The Pokérus doubles the EVs earned from Power items.
In HeartGold and SoulSilver, Professor Elm calls the ☀️ player soon after healing their first Pokémon with the Pokérus to tell 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.
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 Mersenne twister 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).
X values ☀️ Chance to occur 0, 8 0/224
(Cannot naturally occur) 1, 2, 3, 4, 5, 6, 7 31/224
(13.83929%) 9, 10, 11, 12, ☀️ 13, 14, 15 1/224
(0.44643%)
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.
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.
Pokérus is not present in Pokémon: Let's ☀️ Go, Pikachu! and Let's Go, Eevee!.
Generation VIII
In Sword and Shield, a Pokémon's summary screen will show the Pokérus icon along ☀️ with full text saying "POKÉRUS" with the typical smile icon next to the text. When cured, this text is then ☀️ removed and only the cured icon is shown. These icons are shown with the rest of the Pokémon's icons such ☀️ as the origin mark and shiny status. In the PC, this text is removed regardless if the Pokémon is infected ☀️ or cured, only showing the face icon matching the status of infection. It can be seen next to the Pokémon's ☀️ type and shiny status.
In Brilliant Diamond and Shining Pearl, the Pokémon Center nurse will tell the player that their Pokémon ☀️ has been infected with Pokérus like in the original Diamond and Pearl games. The icon for the games is almost ☀️ like how it was in Sword and Shield, except that the infected icon in the PC preview is contained in ☀️ a pink box similar to how it is presented with text in the summary. The infection text in the Pokémon ☀️ summary is written out as "Pokérus", with a capital P.
Pokérus does not appear natively in Pokémon Legends: Arceus. It cannot ☀️ be contracted naturally, nor can infected Pokémon spread the virus to other party members. It also cannot progress towards becoming ☀️ cured, and is not displayed on status screens. However, an infected Pokémon transferred into Legends: Arceus from Pokémon HOME will ☀️ still gain bonus EVs during battle, despite no visible indicators of being infected, cured, or gaining EVs while it remains ☀️ in Legends: Arceus.
Generation IX
In Pokémon Scarlet and Violet, Pokérus no longer generates naturally, does not spread to adjacent party Pokémon, ☀️ cannot be seen on a Pokémon's summary, and does not provide double effort values during battle. However, the data itself ☀️ is retained in the data structure of the Pokémon, and will remain stored with the Pokémon if transferred from previous ☀️ games via Pokémon HOME into Scarlet or Violet, despite no longer having any visible indicator nor in-game effects.
Generation II
"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!"
Generation III
A player being told ☀️ their Pokémon has caught the Pokérus
"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."
Generation IV
"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 ☀️ 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!"
When called ☀️ by the player: "Hello, ?
It seems that Pokémon that have been infected with Pokérus level up better.
We're not quite sure ☀️ why..."
Generations V-VIII
"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."
Status icons
In the anime
In ☀️ Oaknapped!, Dr. Namba explained to Professor Oak 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 manga
Pokémon Adventures
The Pokérus was discussed in The Final Dimensional Duel 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 realize that the infection is, in fact, the Pokérus.
Trivia
The Pokérus is an example of a mutualistic virus, ☀️ in which both host and virus benefit.
In other languages
Language Title Chinese Cantonese 寶可病毒 Poké Behngduhk Mandarin 寶可病毒 / 宝可病毒 Poké ☀️ Bìngdú *
神奇寶貝病毒 Shénqí Bǎobèi Bìngdú * French Pokérus German Pokérus Italian Pokérus Korean 포켓러스 Pokérus Portuguese Pokérus Spanish Pokérus Vietnamese ☀️ Pokévirus
References
