Pokerus is a virus that is very contagious and can spread to Pokemon in your party. It doubles your EV gains, so it is a useful thing to have if you want to EV train. It is temporary.
If a Pokémon has a smiley face, it means that Pokémon is cured from Pokerus. A Pokémon with a smiley face won’t get inflicted with Pokerus again. However, EV gains are still doubled.