Pokerus is a virus that pokemon can get,when you see the pokemon center receptionist she will tell you if you bring your pokemon in for healing.With pokerus a pokemons stats increases at a faster rate,making it easier to train.If you keep that pokemon in your party,the pokerus can be spread to teammates,as well.After a few days,pokerus is no longer contagious and goes away,but its reffect remains.On your pokemon skill page if your pokemon has pokerus itll have a purple colored box with the letters PKRS ight beside where it tells you what level it is,When the pokerus is gone when you go to ur pokemons skill page a smiley face will be on the right side of your pokemons pic.,when your pokemon has a smiley face it can not spread to any other pokemon,if you keep it in ur pc for like 3 yrs. it will still have pokerus.I sujjest that you should spread it to your whole party before time runs out,and pokerus goes away in a few days (life days) not pokemon days.