is it bad? I was gonna use the Pokemon on my team for this game. but if this is a joke put on the game I will just restart. so will it mess anything up?
Pokerus is a good disease that gives you extra evs by doubling the amount gained per each Pokemon. It is good and improves your Pokemons stats. This is extremely rare so you should be happy. The chance of getting a pokerus Pokemon is 3/65536 which is rarer than getting a shiny Pokemon so you should be proud. If you keep this Pokemon in your party, it will spread to other Pokemon so you should do so. Pokerus goes away after awhile so you should train fast to make sure you get the boost.
Its a virus but its a good one. It doesnt harm your Pokemon but it helps them grow. They gain Evs twice as fast now thanks to pokerus. It wont mess up you game in anyway so you dont have to worry about that.