Poké Rus! A Pokémon Disease?

The Pokérus is a useful little virus that can help you while you train your beloved Pokémon. You may or may not have heard of it, but chances are that you haven’t heard of it if you are reading this. You may be wondering “Why do I need it” or “How can a virus help my Pokémon” or “How do I get this ‘Pokérus’” or “Why is this guy asking so many questions”. Save for the last one, it will be covered here.

Let’s start with what it does. The Pokérus is, as mentioned before, very helpful in training your own Pokémon. If you have the Pokérus on a Pokémon, you have probably noticed that it will spread to the other Pokémon in your party and when you level one of those Pokémon up by battling, their stats will increase higher than usual. The reason for this is because the Pokérus doubles the Effort Points you’ve obtained. In other words, it’s just like the Macho Brace you get north of Mauville except without the speed boost and it doesn’t take up an item slot. (For more info on Effort Points, go take a look at the Effort Value guide.) It’s quite helpful indeed. It’s important to note that the Pokémon with the Pokérus will lose the Pokérus eventually, and a small dot will appear in your Pokémon’s status screens. If you want to keep the Pokérus forever, make sure you put an infected Pokémon in your box.

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. You will be alerted if you have the Pokérus in your party when you next visit the Pokémon center. The Pokémon nurse there will go through a dialogue with you dealing with the Pokérus after you get them healed. You can catch it as well by catching the Pokémon and placing it in your party to spread it.

And that is amazingly everything related to the Pokérus…
From Serebii.net