Pokerus is a very useful and VERY rare virus in the Pokemon world. It's rare to get it in the wild, but it's easy to come by now that online trading is so easily accessible. This virus will allow Pokemon to gain Effort Values twice as fast as usual. It's the same thing as using a Macho Brace or other Power item, except the Pokemon doesn't have to hold an item, and their speed will not be cut in half.
Pokerus can be caught by battling a wild Pokemon that has the infection. The virus is about as rare as a Shiny Pokemon in the wild. Once a Pokemon catches it, the word Pokerus will be shown in the Pokemon's summary menu. The Nurse at the Pokemon Center will even inform you about the virus the first time you obtain it.
In order to spread the virus, keep the infected Pokemon in your party around non-infected Pokemon, and start a few battles. After a few battles, atleast one new Pokemon should catch Pokerus. You can try switching around the Pokemon to make sure it spreads quickly.
After one to four days (at midnight), Pokerus will no longer be contagious, but your Pokemon will still have it in their system. It may not say Pokerus at their stat screen, but it will show a small purple :( in the Pokemon's Summary Menu. To make sure you keep Pokerus for a long time to infect future Pokemon, keep an infected Pokemon in the Pokemon Storage System (PC at any Pokemon Center). While in the PC, it will remain contagious until it is taken out for one to four days. The time varies from Pokerus to Pokerus.( This is so you know what PokeRus actually is)
Source : IGN