Walk around, touching every piece of ground to remove the bacteria near to it. It took me some time to realise that the bacteria grows back after you've removed it, meaning that you end up going over and over the same bits of the level trying to get it all clean before the first bit you did grows back; rather like painting the Forth Bridge, where once you get to the end you have to start all over again at the beginning. And once you get a few levels in, there are enemies that fire bullets which kill you (which is fine) but you walk faster than the bullets fly, so if you insert yourself into a bullet line and walk along with it, you end up walking into the bullet in front of you (and dying), which is pretty annoying. Although that's likely my fault because I'm not good at this sort of twitch reactions game. The implementation is flawless as far as I can tell, graphics and music and all.