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.
Beijing Subway, China's oldest metro system, currently ranks as the world's third longest subway system comprising of 17 operatinglines spanning a total length of 456km. The subway system has been in operation since 1969 and is owned by the city of Beijing.