Pokérus is a beneficial virus that can randomly appear as a Status condition on a Pokémon, with a 1 chance and a spread rate of 1 by default. A wild Pokémon can also contract this disease, but it cannot be seen until that Pokémon is caught.
You can tell if a Pokémon has Pokérus by viewing the Status field on the Summary page, which will show a purple box labeled PkRS if the Pokémon is infected.
Pokérus has the effect of doubling any EV yields that a Pokémon receives. This effect is cumulative with the Macho Brace, which then gives you a total of 4 EVs for every Pokémon defeated. If a carrier is in the party, the virus may spread to other party Pokémon, eggs included.
The virus automatically expires after timespan of 1-4 real days, depending on the strain contracted. Once expired it will no longer spread, although its EV-boosting effects will remain.