The Pokérus (Japanese: ポケルス Pokérus), from "Pokémon virus", is a microscopic life-form that may attach to Pokémon, first appearing in 2️⃣ the Generation II games. It is a beneficial mechanic that a Pokémon can obtain.
In the games
When a Pokémon has the 2️⃣ Pokérus, it gains double the effort values from battling (e.g., fighting a Magikarp will give two Speed EVs, rather than 2️⃣ one). Effort points gained from Vitamins and Feathers are not doubled. The infected Pokémon can infect other Pokémon with the 2️⃣ virus for a period of one to four days. However, the Pokérus timer can be delayed by several methods, such 2️⃣ as by placing the infected Pokémon in a PC Box.
While it is represented similarly to status conditions, the Pokérus is 2️⃣ not a status condition, so it cannot be healed at a Pokémon Center or with any status condition healing item.
Pokérus 2️⃣ has a 3 in 65,536 (or approximately a 1 in 21,845) chance of being generated on one of the player's 2️⃣ Pokémon after a battle, making it rarer than encountering or hatching a Shiny Pokémon. If any of the player's Pokémon 2️⃣ are infected, the Pokérus may spread among Pokémon in the player's party after any battle.