Pokerus is an extremely rare condition that is only generated on about 1 in every 21,845 wild or bred Pokémon, making it more rare than encountering a shiny Pokémon!
Pokerus doubles the amount of EVs your Pokémon will gain in a battle! You first got the condition when you were battling a wild Pokémon that had the condition. After defeating said Pokémon, the Pokémon that you were using to battle was also "infected"
After one of your Pokémon was infected the team member(s) directly next to the infected one received Pokerus after a battle until it was spread to all of your team.
After a certain period of time the Pokémon in your party will lose the "pkrs" symbol and Pokerus will no longer be contagious, but the already infected Pokémon will retain the effect of gaining double EVs. You probably want to make sure you can continue to transfer this to other Pokémon in the future so if that is the case you can simply put one of your infected (and still contagious) Pokémon into a PC box until you want to spread it again.
Hope this helped.