This game was the first time in my life that I found myself saying, "Heck yeah, necrosis!" It's a fantastic concept for a game: You control an epidemic and your aim is to spread it throughout the world and kill everyone before humanity can develop a cure. You have a variety of tools at your disposal to mutate your virus: the ability to add symptoms, including fatal ones; methods of communicability, including animal borne, airborne and body fluids; and resistances.
Each of these can be built up in trees that interconnect, making your virus strong and, as your virus spreads, you gain DNA points that you can spend on more abilities. You can watch the effects in a newsfeed, such as "Australia burning corpses" and "France removes drug research safeguards". It's tremendously exciting, especially when your virus grows strong enough to mutate on its own, as you race against the development of a cure. It's based on a real-world simulation, too.
Destroying all humans has never been so much fun.