Plague Inc is an awesome iOS game that involves you controlling different diseases, including viruses, fungi, parasites, prions, and fictional diseases, such as neurax worms, and necroa viruses. When you start, you choose what country to infect, and with
DNA points, you can upgrade transmissions, symptoms, and abilities to try to infect and kill every
human on Earth. It has generally pretty
popular, on the App Store and extremely fun.