Influenza attacks and this game you are the white blood cell and your goal is clear - destroy the central virus. In every level you face a boss every time. But he is not a week guy and produces lines of viruses. You can touch them but their shots are dangerous for you. For destroyed enemies you get experience and fro them buy some upgrades, for example better stamina. In higher levels the viruses are stronger and stronger so every special skills of the white cell is for sure needed. It is up to you what skill tree you choose and what type of blood cells you meet.