I would like to remind everyone that fast infected are quite common in our roleplay. So far i'm only seeing posts including the slow infected. Fast infected can sprint, faster then you even. And they can scale small obstacles and climb up things faster. 

Basically, even entering a store can become a clusterfuck if you encounter one or two of the fast ones. Personally I think I could kill a normal walker in real life, but if one of them comes sprinting at me then i'm going to need a change of pants. :P