Here's how to get Pokerus in FireRed:
1. It's all about chance. Pokerus is a random encounter. You can't force it to appear.
2. Keep an eye on your Pokémon's status. After a battle, if your Pokémon has a pink "PS" icon next to its name, that's Pokerus!
3. It's contagious. If a Pokémon with Pokerus is in your party, other Pokémon have a chance of catching it too.
4. The benefits: Pokerus doubles the amount of EVs your Pokémon gains from battles. This is extremely useful for competitive battling.
5. It lasts for 24 hours. After that, the virus disappears, but your Pokémon will still have the benefit of doubled EV gains for the rest of its life.
Key things to remember:
* Pokerus is rare. You might need to battle a lot of wild Pokémon before you encounter it.
* The PS icon disappears after 24 hours, even though the effect stays.
* You can spread Pokerus to other Pokémon by having a Pokémon with it in your party.
Good luck finding Pokerus in your FireRed adventure!