pokerus

Basically you get pokerus via the same method as getting a shiny Pokemon, totally random luck.
EV training is done to increase the value of a particular stat of a given pokemon. Each wild or battlable Pokemon gives the Pokemon who helped in defeating it a fixed number of EV's based on what type of Pokemon was defeated. EVs are obviously categorized into HP, attack, special attack, special defense, defense and speed. A given Pokemon could give as many as 3 EVs when it is defeated. Now as your Pokemon racks up these EVs by defeating Pokemon it's stats increase. For every 4 EVs gained in a particular stat category the value of the stat increases by one. This is gained on the next level grown.
Heres an example. My metagross is level 55, but was caught at level 53. I took it west of pastoria city and defeated many many bibarels and kricketunes. When it reached level 56 it's attack stat changed by +13! That is because for each kriketune or bibarel 2 EVs are awarded to Metagross. Obviously there is a limit to how many EVs can be obtained. 255 EVs for any particular category and 510 for the whole pokemon. The key to EV training is deciding the final moveset for your Pokemon early on based on it's nature and deciding where the EVs shuold be allocated. Patience is also helpful. Pokerus and the macho brace are also helpful as both increase the amount of EVs obtained by double. Combine the machobrace with pokerus together and get 4x the EVs hurray!
A few side notes... Make absolutely sure to get all the EVs in before level 100. This can be checked in the ball capsule place in sunyshore city. If you get an effort ribbon you have full EVs. EVs in unwanted categories can be removed by certain specific berries. I think that just about covers everything you could need to know about EVs

Pokerus is a virus or an infection that one of your Pokemon can randomly be infected with. You would probably never know if one of your Pokemon were infected with the Pokerus unless you were looking for the sings specifically. You can tell if one of your Pokemon is infected with the virus by looking at the summary page of the Pokemon, or by taking your Pokemon to the Pokemon Center to be healed. However, unlike other viruses which are bad, Pokerus is actually a good thing for your Pokemon. You see, whenever a Pokemon is infected with Pokerus their EV points earned will double. What this means is that whenever they level up, there stats will go up more than they usually would. EV training is leveling up a certain stat of a Pokemon. Like, only levelingt up their attack, or their defense, something like that. You can find EV guides just about anywhere, search Google.


Hope this helps.