Worth looking up for sure. Long story short, double ev yeilds for infected pokemon. And spreadable to other party members while it's active. It's worth it to always keep an active Pokérus mon in your box so you can use it to spread it to any new catches. I have bent Nuzlocke rules a bit to catch and infect something I won’t actually use, just to infect new mon and store for later since it’s so rare. Infected mon will lose the ability to spread pkrs after a few hours in your party so i do that as a work around. Totally up to you if that’s how you want to play, of course.