The infection may spread if the infected Pokémon is in the main party after a battle. An uninfected Pokémon must be next to a Pokémon infected with Pokérus in the main party for the virus to spread. If Pokémon that have already been infected and cured from Pokérus are in the main party and are on either side of a Pokémon currently with the disease, Pokérus will not spread after any amount of battles. Source.
You must battle to spread the PokeRus, but not necessarily with the PokeRus Pokemon. Source: Above Link
if you want to spread it amongst your Pokemon, you have to battle with a pokerus Pokemon and the rest of the Pokemon in your party should get infected. pokerus has no negative effects.