When you first start out in the game, there will only be one plague type available: Bacteria. The 6 other plague types can be unlocked in progression. The order of unlocking is as follows: Bacteria -> Virus -> Fungus -> Parasite -> Prion -> Nano-Virus -> Bio-Weapon. Unlocking the Neurax Worm is more demanding, though. All plague types can unlocked through in-app purchases. Note that beating any game on casual difficulty will not unlock the next plague type.