When a Pokémon contracts Pokerus, it enters the active stage, during which it displays several symptoms. These symptoms include a slight rise in all of the Pokémon's base stats and a doubled rate of Effort Values (EVs) gained after battling. This doubled EV gain is the most notable and helpful aspect of Pokerus, allowing players to quickly train their Pokémon's stats.
After a certain number of battles (usually 1-4), the Pokerus will enter the dormant stage, where the symptoms disappear. However, the Pokémon will still be able to spread the Pokerus to other Pokémon in its party, albeit at a reduced chance.
As long as at least one Pokémon in a party is infected with active Pokerus, the chance of contagion remains, spreading quickly through the entire team over time. Once all Pokémon in a party have contracted Pokerus, there is no longer any chance of further contagion.
Once dormant, Pokerus can be cured. When using the Heal option at a Pokémon Center or using a Full Heal, the Pokerus will be eliminated entirely, including the doubled EV rate and contagion abilities.
Pokerus is a rare and beneficial occurrence, assisting trainers in raising powerful Pokémon. If you're lucky enough to encounter it during your journey, you should infect your entire party for the stat-boosting benefits it provides.