I had this neat arcade game the other day. It plays like Tyrian in Arcade Mode, except horizontal and scrolls from the right. Basically, you're a white blood cell in some sick guy's circulatory system, and you have to fight off pathogens. At first you have a simple pea-shooter lysosome gun, but you can get better guns, including an antibody cannon, a viral manipulator engine (shoots virii at bacteria, reprogramming them to commit suicide), and a phagocyte gun that shoots a huge blob that swallows all the pathogens on the screen, a la Pacman.

At first you just kill bacteria and pollen, then you run into cancerous cells that you have to kill using lysosome guns, then you fight virii which turn other blood cells against you.

Haven't worked out the details yet, but I thought it was cool. What do you think? =D