Quarantine leaves a bit to be desired with only six scenarios and three difficulties as well as its lack of multiplayer, but that doesn't mean it's a bad game.  It's definitely challenging on the higher difficulties as the viruses start out a little stronger.  However, if you're looking for a good challenge, Quarantine can definitely give it to you, plus the custom scenarios on the Steam Workshop will help keep things fresh…or at least as fresh as they can be with the same gameplay.