Have you ever played
Pandemic or Pandemic 2? If so, you might appreciate "Plague Inc" from
NDemic Creations.
It's a sort of strategy game where you play as different plague types (Bacteria, Virus, Parasite, Fungus, Prion, Bioweapon etc etc) to infect and ultimately kill off all life on Earth. It's a morbid idea and it definitely manages to get your skin crawling as you play, but it's oddly addicting and well made.
The game features in-app purchases to buy new disease types and "genes" to modify your genetic code for small bonuses. All of the genes and disease types can be unlocked simply by beating the game once with every disease, so it's not like you have to purchase it. I think the Android version have to be "unlocked" by a single purchase in order to modify the gene code, but it's otherwise free to play (ad supported unless unlocked), whereas the iOS version is not free and thus is unlocked on purchase. There are also one expansion out already (adding the Neurax Worm disease type) and more on the way.
Give it a whirl on either the
iOS app store or
Google Play.
I heartily recommend it, especially since the guy (yes, a single dude) is really nice and is one of few iOS devs that despise the "customer cash-cow" milking scheme with horrible in-app purchasing schemes.