Part 46: Mini Note 05: Poke R USMini Notes 05: Poke R Us
Something I've alluded to in the past, now I've finally obtained it for myself, how lucky. Pokerus is very beneficial to Pokemon that contract it, but while the chances of it can be forced easily enough, it's still incredibly rare.
It's a 1 in 21,845 chance (0.000046%) of being generated on a Pokemon after battle, either through battling or catching, running away will not contract it. A Pokemon with Pokerus will also infect other adjacent Pokemon in the party after a battle, so once it's obtained, it'll spread very quickly. There was quite a long gap between Pokemon Center visits, so we could've gotten it from anything in Mt. Coronet, wild Pokemon, Team Galactic, even Giratina, but to burst your bubble, none of the wild Pokemon we caught have it, so it wasn't from any of them. Sorry. So why keep this virus around? Well, it doubles all EV gains from battles, not from vitamins or wings, but do include the power items, so it can make EV training a breeze. It's definitely truly rare in Gen 2 and 3, but starting in 4, with the advent of online trading, getting it becomes inevitable.
Depending on which of the four strains you obtain depends on how long the Pokemon has the virus, 1-4 days, this value will only decrease if it is in your party when a new day starts, so keeping an infected Pokemon in the PC or the Day Care lets you keep it indefinitely to spread to anything you want. It can even spread to eggs, starting in Gen 3. Once the Pokemon is cured, it can never contract the virus again, but it still gets the benefits, so it's worth infecting as many Pokemon as possible, the fact that it's on my main team is just nuts, as if they weren't already ridiculous strong! It'll state pretty clearly if a Pokemon has Pokerus, it'll appear as a status condition on the summary screen, and when it's cured, it's a little face, or a dot in Gens 2 and 3.
Speaking of those gens, Gen 2 is when it was introduced, so if you trade a Pokemon back to Gen 1, all traces of Pokerus disappear since the data isn't stored in any way, letting it be infected again when you trade it back. Pokerus isn't in FRLG or ColoXD due to no time of day, making it much rarer. And that's every to do with this little virus, it stays pretty much the same across the entire series, so I'm sure you'll get it one day. Or just keep trading for it, it's much easier nowadays.