Contagion is a fitting topic for an iPhone game, considering how often I've picked one up after coming in contact with someone that already had it. And though Tiny Invaders isn't a deliberate metaphor for viral marketing, I'm happy to help spread its icky green tendrils.
First-time developer Hogrocket (which is basically cheating by employing Geometry Wars designer Stephen Cakebread) has created a fun little game about trains, and then hidden it inside a lighthearted tale of alien invasion. In order to take over the bodies of various American caricatures, you guide your microscopic, autonomous invaders through the bloodstream by tapping on turns and junctions to change their path. Once you've collected all the on-track orbs and carried them back to the originating ship, you move on to the next stage.