It's not that it's easier to catch in your own game but, due to wireless trading and the GTS, it's a hell of a lot easier to distribute it to other people.
When you think about it, it only requires one person to get an infected Pokémon, then that Pokémon spreads the virus to other Pokémon and the person trades those Pokémon to other people that follow the same method. Pretty soon every person who has WiFi will have/had the Pokérus virus.