The Virus was a villain and a computer virus from SMG4's new computer who took over the whole castle.
After SMG4 released the virus instead of destroying it, it consumed the whole castle into some glitch world. SMG4 saw the computer inside a black hole in the computer room and jumped inside, along with Mario. After getting deeper and deeper into the glitch, SMG4 finally encounters the virus itself. He realizes the computer can get rid of the virus but fails, however Mario crushes the virus with some random machine he found inside of the glitch world.
- The Virus shares a strikingly similar sprite as Missingno., a well-known glitch in the Pokémon universe.