The Pokerus

Let me guess. You have no idea whatsoever what the Pokerus is. It's actually a virus that Pokemon can catch. Don't panic, don't worry, it's good. See, when a Pokemon with Pokerus is in battle, it gains double the EVs as normal. For a Pokemon to get the Pokerus, it will either catch it from another Pokemon with Pokerus, (both have to be in your party,) will be caught with it, (rare,) or it will just develop it. To know if a Pokemon has it, check its summary. If there is a thing saying PKRS, like a status condition, the Pokemon has it. Eventually, a Pokemon will recover, unless it is left in a Box. When a Pokemon recovers, you will see a small purple dot next to it on your profile.