How does the pokerus spread between Pokemon in the party?

Question

Like do we have to battle a lot, walk around, catch Pokemon or sumthing, or does it happen as time passes.

Answer 1

>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.

Just put the Pokemon you want to infect next to the Pokemon infected, and then battle.

Source