The Pokerus is a useful virus that infects and attaches to a Pokemon. It can increase it stats and double the Effort Values of a Pokemon when in battle. While infected with the Pokerus, the Pokemon grows at a faster rate and is easier to train.
The Pokerus virus is rare, and obtaining one involves encountering a Pokemon who is already infected. Once infected, it will stay with the Pokemon for up to 4 days before it wears off, and can be spread through battling uninfected Pokemon as well as through training. This extraordinary virus is advantageous to the strategy of the game, and if one Pokemon is infected, it is wise to infect the rest of the Pokemon through battling and training, which will increase their stats.