A virus is a microscopic organism that can replicate only inside the cells of a host organism. Most viruses are so tiny they are only observable with at least a conventional optical microscope.
Recognized by the Guiness Book of World Records as the MMO with the longest play time per player, and easily the oldest game on this list (launching four years ahead of World of Warcraft, way back in 2001), Runescape deserves mention because it's one of the most updated and popular games of any kind on the planet.
GWR Video Gamer's Edition 2008 record updated and featured in GWRGE 2010 It is beaten to the record of overall longest-running videogame franchise by Space Invaders, which was first released in 1978 and most recently updated with Space Invaders Extreme 2 in 2009.