It's not simple to be frank, and also unlikely that you'll ever get a Pokemon w/ Pokerus.
So now how do I get this elusive virus? Well to tell you the truth… It’s really based upon luck, like most things. Maybe not as much as to get a shiny Pokémon, but around that area. To get the Pokérus, your Pokémon needs to engage in a battle a wild Pokémon that holds the Pokérus. -Serebii