Pokerus doubles your EV gain while you have it. I thought they'd gotten rid of it! Wow. :o
EV stands for “Effort Value.” They determine what stats your Pokémon gets upon levelling up and are gained from battling any Pokémon you come across that award Experience points. Each stat is given +1 for every 4 EVs you put into it. The maximum amount of EVs you can give a stat is 255.
Beating a Patrat would give all Pokémon that participated in the battle +1 Effort Value to Attack.
Beating a Frillish would give all Pokémon that participated in the battle +1 Effort Value to Sp. Defence.
Each Pokemon awards a different type of EV, so you can look here to find a good list in SM: https://www.serebii.net/pokedex-sm/ev/hp.shtml (click across the top to see the other stats)
255 / 4 = 63.75
Obviously a stat can't have .75 added to it, so this is rounded off by the game down to 63, which is the same addition you'd get from 252 EVs. Therefore you're wasting 3 EVs if you put anymore than 252 into any one stat. Simple right? Okay, now lets apply this to the fact you can have 510 EVs on a single Pokemon.
510 / 4 = 127.5
Because the maximum amount a stat can be added to is 63, you end up with 1.5 of a possible stat left over. Again, .5 of a stat doesn't exist in the game so this is rounded down to 1 being left over. Therefore no matter what you do 2 of your 510 of EVs are going to be wasted. If we look at my Volcarona again you'll notice it has 252/252/4, equivalent to 508 effective EVs.
With all this in mind we can say in simple terms; Don't put more than 252 EVs into any stat.