Warning: Portal 2 is a virus and it’s infecting many of the Potato Sack sale games on Steam. [Fair Warning: If you don't use Steam, this all might sound like gibberish.] The Ball contracted the disease, and after a free DLC it developed a whole new level named Aperture, which gives an hour of new gameplay. Audiosurf began exhibiting symptoms as well, with Portal 2 themed levels and artwork.  Even the zombie-slaying game Killing Floor wasn’t immune to the outbreak, as it too spawned a new Portal 2 level.  So far the only treatment for the disease is to remain indoors, download all of the Potato Sack games on sale on Steam, and play Portal-inspired levels while you wait for the actual game on April 21—though some rumors say it may release early! [via Big Download]