Pokérus has a 3 in 65,536 chance of being generated on a Pokémon, either wild or bred.
So yeah, its not easy to get the pokerus. But you are in luck, as the pokerus IS able to be spread. to be spread, the Pokemon must have an icon in its summary page that looks like This. To spread it, take your pokerus Pokemon and any other Pokemons that dont have it and battle wild Pokemons(it usually takes 1-5 tries). Afer every battle, the Pokemon NEXT to the Pokemon with Pokerus might be infected. The best places to put the Pokemon with pokerus is in your third party slot or your fourth party slot. There ya go!
Make sure not to have any pokerus-infected Pokemons in your party around midnight (doesn't matter if your ds is on or off), as the ability to spread pokerus will go away. Put them in the PC and they will be "safe" from losing the pokerus.
Note: Make sure you have a backup spreader, in case you fall asleep while playing, or something like that.
Source:Bulbapedia and My experience