The Pokérus, short for Pokémon virus, is a rare affliction that can appear on a user's hatched Pokémon. Every day, a small number of Pokémon are infected at random. This disease causes maturity gained from interactions and views to increase; however, the infected Pokémon cannot be moved from the user's party at all, not even to release it. Under normal circumstances, the disease lasts 3 days, during which it can be passed to any other hatched Pokémon in another user's party. An Antidote can remove the disease before the 3-day limit. Any Pokémon that is cured of the disease, either "naturally" or through the use of an Antidote, returns to regular maturity gains and cannot be infected again.
In May 2010, changes were made in the way Pokérus could be transmitted. Pokémon coming back from the Underground have a small chance of returning with the disease. Interacting with a user's infected Pokémon has a chance of spreading the disease to the interacter's Pokémon.
Pokémon adopted from the Safari Zone may also come infected with Pokérus.