Actually, DontClickOnMe is somewhat correct, a virus to cause them to move slowly is mainly caused by their brain being unable to function right, low dexterity, since zombies basicly have no stamina or anything... they will run until their muscles are literally nothing.
Alright, this game is realy... crappy! but im trying not to sound cruel, realy... I am! but it's total crap, you realy should work on your programming skills before releasing another game.