The first time a Pokémon Center nurse tells you that your Pokémon is carrying a virus, the Pokérus, panic looms. There is no reason to worry, though: introduced in Gold and Silver, the Pokérus is the only virus that makes your Pokémon stronger.
What is the Pokérus?
The Pokérus is a microscopic virus that attaches itself to a Pokémon. As long as it is infected, the Pokémon gains twice as much Stat Exp. from every battle: those hidden points that raise its stats on top of its level. An infected Pokémon therefore grows stronger, faster, than another one trained the same way. It has no negative effects.
How to catch it
The Pokérus is passed on at random during battles against wild Pokémon or Trainers, with an extremely low probability (around three chances in 65,536 per battle). There is no way to trigger it on purpose. You will know that one of your Pokémon is a carrier when a Pokémon Center nurse tells you so, or by checking the Pokémon’s summary.
Spreading it to the whole team
After each battle, a carrier can infect its direct neighbors in the party. To infect everyone quickly, put the carrier in the middle of the party and fight a few battles, then change the order.
Recovery and keeping it
The Pokérus goes away by itself after a few days (between one and four), counted each time the clock passes midnight. Once cured, the Pokémon cannot be reinfected but keeps the Stat Exp. bonus it has already earned; it no longer spreads the virus.
