Okay, keep it in your party for awhile and the others will catch it and I think if I remember correctly that if you want to keep that pokemon with pokerus then put it in the boxes. If you look in the games mechanics on the main site, it will have a page about pokerus.
Switch all infected pokemon in the team except for one to the poemon you want to get infected. Then hang around until all of them has the virus. I have infected three entire save files main teams plus some legendaries. Oops, i searched for pokerus, i didnt see that this was the G/S/C discussion. I am talking about sapphire.