You just have to be lucky and battle a wild Pokemon with it. If you manage to find a wild Pokemon with the virus, it can spread to your own Pokemon and then through your party, but the chances of finding Pokerus are even smaller than finding a Shiny Pokemon, so the virus is really rare.
My sister found one and I was so jealous. So I tried to get one myself, and realived that I wasn't gonna find one so I got her to trade the pokemon with the virus to me for a level 50 unknown. (I had LG)
Click on Dragonite and take the personality test!I'm BULBAFIED!!!