Instructions
Battle a wild Pokemon with the Pokerus. Pokerus appears on wild Pokemon completely at random and there is no indicator that a wild Pokemon has the disease. You will find out whether or not you have the disease next time you visit a Pokemon Center, where Nurse Joy will inform you that your Pokemon has it.
Catch a wild Pokemon with Pokerus. Again, the condition appears randomly and has a very low chance of appearing on a wild Pokemon. The odds of getting Pokerus in the wild are extremely low.
Put the infected Pokemon in your party to spread the disease to other Pokemon. Switch any other Pokemon currently in a box on your PC into your active party to infect them as well.
Trade for infected Pokemon on the Global Trade Station terminal in Jubilife City. Some trainers consider it courteous to infect Pokemon before they trade them away, so you may luck out and get the Pokerus that way.