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.
One oft-cited candidate for the worldâs longest metro network is the one in Seoul, which only opened in 1974 but by 2013 included 987.5km of route on 18 lines. That's pretty much enough to get you from London to Marseille. No other city comes close.