I tried for a few times, but they still didn't get Pokerus. There's a pink smiley face so they had Pokerus before. Can I give them again so that I can use them to spread to other Pokemon?
>Once a Pokémon's immune system has fought off the virus, they cannot spread it further, nor can it be spread to them by other Pokémon. -Bulbapedia
You cannot use it to spread Pokérus to others, but it can still get the double EVs boost.
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 to or through them after any amount of battles.
Bulbapedia