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.
It is in particular these type of free-to-play games that dominate todayâs online games market, thanks to their appeal to players in emerging markets like China. The highest-grossing subscription-based MMOs are older titles like World of WarCraft that continue to retain longtime players.