Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
Welcome to PokéCommunity! Register now and join one of the best fan communities on the 'net to talk Pokémon and more! We are not affiliated with The Pokémon Company or Nintendo.
Use these commands in your script:
...
setvar 0x8004 0x0 //0x0 is for the 1st pokemon in the party, 0x1 is for the 2nd pokemon...
callasm 0x8?????? //offset of the decrypt routine (replace the ? by a free space offset)
setvar 0x8005 0xF //0xf is the maximum duration of the virus (4 days I think...)
callasm 0x8?????? //offset of the give_pokerus routine
callasm 0x8?????? //offset of the encrypt routine
...
Now, you put the machine code of the routines at the offsets you choose: