So, I originally played this game on PC and moved to mobile when I learned I could play it anywhere. It's a great app, and really cheap for what you get. Now, there are a few pay items, such as the scenario packs and now the scenario creator, but the rest of the content is locked behind gameplay, which is great! You also have the option to pay for the content, instead of progressively unlocking it by beating each plague type, but that is not necessary. You will need to play to win though by devising good strategies for each plague type, as they really are all different. You can unlock the standard plague types by simply beating them in in order, but after that there are the special types to unlock and of course the cheats, which can make the game extra fun just to mess around with. Unlocking these is a challenge, especially the last ones where you need to beat all standard plague types on Mega Brutal, but it's definitely doable. I've done so on both my iPad and iPhone, but you will likely need to try multiple times for each type in order to devise a great strategy to win against the really tough conditions of Mega Brutal. The hardest for me in all difficulties was the Nano-Virus, as the cure begins right at the start of the game, but even that can be won with persistence. It's not a chore, it's fun to learn how best to evolve and disrupt humanity in order to infect and kill the world. So enjoy the game; it's a great price and if you find it fun enough you can expand it with the scenarios pack and even the scenario creator to keep the fun going without breaking the bank.