the Pokémon Virus (aka Pokérus) is a virus that infects your Pokémon. but its not a harmful virus, it raises there stats much higher when you train them (so its a very good thing). if you use the infected Pokémon, it can infect the other Pokémon on your team (which leaves you with a very good edged t raising your Pokémon). so, in short, the Pokérus is good thing, not a bad thing.
Just to point out, The pokerus raises the total EV gain, by doubling it, so you get 2 speed EVs from a golbat, if a pokemon has pkrs it'll get 4.