This script allows for battlers to have a switch in their data that says if they have pokerus or not. I did not add the effects but I know two scripts that can do the job.
What you put in for the condition is: infected?
What is Pokérus?
From the Bulbapedia page on Pokérus
“Unlike other status ailments, Pokérus cannot be healed at a Pokémon Center or with any status ailment healing item, and can only be obtained by the Pokémon in question once. When a Pokémon has the Pokérus, it gains double the effort values from battling (i.e., fighting a Magikarp will give two Speed EVs, rather than one). Effort points gained from vitamins and wingsare not doubled. The infected Pokémon can infect other Pokémon with the virus for a period of about two days. However, the Pokérus timer can be delayed by several methods including placing the infected Pokémon in a PC Box.
Pokérus has a 3 in 65,536 chance of being generated on a Pokémon, either wild or bred. Through battling or capturing this wild Pokémon, players could get this helpful virus onto their own Pokémon, and subsequently spread it around. Due to the overwhelmingly low chance of encountering the virus (about 1/3 of the chance to encounter a Shiny Pokémon), and the fact that most would not even recognize a Pokémon with the virus and may flee from it, many players would never have a Pokémon with the virus.”
Script Download and Requirments
Required: Victor Engine – Basic Module