Each Pokemon gives a certain amount of EVs for a certain stat, the Power items add 4 additional EVs to their corresponding stat(Power bracer gives 4 additional EVs in attack and so on).
Pokerus doubles the EVs gained, so if you fought a Pokemon who yielded 1 EV in Attack, gave your Pokemon a Power bracer and if it had pokerus, you would gain a total of 10 EVs in that stat(1+4=5, 5*2=10).
Here is a list of good EV training spots in Kalos and a more in depth explanation on EV training.
Source: The links above
Hope I helped.