THE POKEMON VIRUS

Pokerus was first introduced in Pokemon Gold, Silver and Crystal. This virus, perceived by some to be a bad thing, is in fact beneficial to the growth of your Pokemon. In GSC, Pokerus doubled the amount of Stat Exp received at the end of battle, meaning your stats grew twice as fast. Now in RSE and LGFR, the virus will double the amount of EVs that your Pokemon earn.

Pokerus will be caught randomly by one of your Pokemon after beating a wild Pokemon. When you next visit a Pokemon Center, Nurse Joy will inform you that your Pokemon has been infected:

Your Pokemon will continue to have Pokerus for around three days. During this time, it will spread to other Pokemon in your party. If you deposit an infected Pokemon in the PC, it will pause the rate at which the virus disappears, but it will not infect other Pokemon in your PC. On the status screen, a small "pkRS" tab will appear to show that your Pokemon is currently infected with Pokerus.

Once Pokerus disappears, your Pokemon will not catch it again (Just like a real viral immunity). A small dot will appear under your Pokemon's level (in Ruby, Sapphire and Emerald) or beside the Pokeball (in FireRed and LeafGreen) to show this. Though this Pokemon can no longer spread Pokerus, it still receives the doubled EVs like it did before.