It appears randomly like shiny Pokemon. It is like a status and is obtainable by catching a wild Pokemon that is infected with Pokerus. To spread it, simply put it into your party and all the Pokemon in your party will be infected in no time.
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The ruby and sapphire are on fire red and leaf green don't worry about the in ruby sapphire or emerald.
No, you have to migrate it from Pokemon Emerald, Ruby or Sapphire. No, you have to migrate it from Pokemon Emerald, Ruby or Sapphire.
ruby, sapphire, emerald, fire red, leaf green
Ruby/Sapphire/Leaf Green/Fire Red/and i think Emerald. if not, trade from emerald to Ruby/Sapphire, and from ruby/sapphire to leaf green. You need to have the Sapphire and Ruby before you trade. you get these after the elite four.
You can do this after you find the Ruby and the Sapphire for Celio in Pokemon FireRed. then you can trade with Ruby, Sapphire and Emerald.