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.
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.)
So now how do I get this elusive virus? Well to tell you the truth… It's really based upon luck, like most things. Even rarer than Shiny Pokémon, there's a 1 in 21,178.6 chance of getting it. To get the Pokérus, your Pokémon needs to engage in a battle a wild Pokémon that holds the Pokérus. Simply battling or capturing this Pokémon will allow for the virus to appear. Even running will work. You will be alerted if you have the Pokérus in your party when you next visit the Pokémon Centre. 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.
The duration that PokéRus lasts is dependant upon a random number, known to some as the strain of the virus and can last anywhere from 1 to 4 days. This gives ample time to spread the virus to other Pokémon. When the PokéRus has gone through the specific amount of time that the strain allows it, the Pokémon will no longer be able to spread PokéRus and it will now have a face on the status screen. However, despite it not being contagious, as of Diamond, Pearl & Platinum, it will still receive the boost of Effort Values so it will forever be good. If you want to keep it being contagious, however, you can keep it in your box and remove it whenever you wish to spread it.
And that is amazingly everything related to the Pokérus…