I suck at contest so I thought if I get pokerus my contest stats go to max power
You get it from random wild Pokemon. However, it is very rare. You have about an equal chance of getting a shiny Pokemon as getting Pokerus. It doesn't help contest stats.
Just keep fighting wild pokemon and to find out is to check your pokemon's summary if there is a PKRS on the status it has it, remember if one has it in the party it will spread to the entire party. . .and remember it's hard to get it
hope this helps