I don't know what is it
It is PokeRus.
PokeRus doubles EV gains. You can spread it, and Pokemon can only have it for a few days, unless it is in the PC.
PkRs is short for pokerus. It is a virius but it is actully helpful because it doubles the evs you get after every battle
PKRS means PokeRus, its where any pokemon who has it gains EVs faster. The ability to spread it goes away after sometime but it will always keep the effects from PokeRus.