Pokerus is a disease you can catch by batteling another Pokemon Basically if you make contact with that Pokemon you will catch pokerus which doubles evs the chance of finding a Pokemon with pokerus is a 1/3 of a chance catching a shiny according to bulbapedia If you have pokerus in your team it will spread after a while
It's completely random and is slightly more common then shiny pokemon and seem to appear only once in each game.
