Plague Inc!!
It's the only smartphone game I really play, hahaha.
It is very smart and very entertaining.
Basically you have to create a disease (bacteria or virus or parasite or pryon or bioweapon... etc...),
and evolve it (transmission, symptoms, abilities) to destroy humanity before they have time to create a cure.
Depending on where you start your disease the climate, transportation, economy and news events will affect it.
It's SO much fun!
They recently released Plague Inc Evolved that I have yet to try.