Pokérus is a very useful thing that is used in EV training. (For a guide on EV training, please click here). What does it do? Well, Pokérus doubles the amount of EVs received from a battle.

This is particularly useful, as EV training is quite a long-winded process. With this handy little virus, the amount of  EVs gained can be doubled, or even quadrupled if you have the right items!

Introduction

Pokérus has essentially the same function the item Macho Brace. It doubles the amount EVs gained from a battle. But because Pokérus is not an item, the Pokémon can have both a Macho Brace and Pokérus, which results in four times the amount of EVs But the best thing about Pokérus is it doesn't have the speed decrease of Macho Brace, and it can be passed on to other Pokémon!
 

Now how useful is that? You don't need to spend loads of money on items for EV training, because you have Pokérus that's easily transferable among your Pokémon! The downside is that it is extremely hard to contract Pokérus in the first place. Of course you can trade with someone who has Pokémon that already has Pokérus. But, before you worry about this, you can check if your Pokémon already has the virus! 

 
To find out if you have a Pokérus infected Pokémon, check the summary page of the Pokémon. There will be a purple or symbol where status problems, such as Poison, Burn, etc., usually are. Another way to figure this out is to go to the Pokémon Center. The first time you go to the Pokémon Center after the Pokémon has been infected, the nurse will tell you that your Pokémon, indeed, has been infected. Look at the picture above for more details. If none of your Pokémon have the in their summary, and the nurse doesn't say anything out of the ordinary, then you're out of luck. Read the next section to find out how you can get Pokérus.

The initial Pokérus infection can come in two ways: wild Pokémon battles and trading. The chances of getting Pokérus from a wild Pokémon are slimmer than finding a shiny. Chances are, you'll never encounter Pokérus in your game if you don't cheat or trade.

If you do battle a wild Pokémon that has Pokérus, there is 1/4 chance that the wild Pokémon will transfer its Pokérus to your Pokémon.

Pokérus can be transferred from one Pokémon to the another if the infected Pokémon is in your party, and a couple of wild Pokémon battles take place. For each turn of the battle, there is a 1/4 chance that the virus will spread to other party members. Note that putting an infected Pokémon in the box will not transfer it to other Pokémon.

A word of caution! Pokérus will stop being infectious after 24 hours. The effects will remain, but it can't be spread any more. You will know this when you see a . or a symbol when you go to the Pokémon's status page. A solution to this is to keep a Pokémon inside the PC box, as the countdown for Pokérus temporarily stops while the Pokémon is in the box.