it is not a matter of getting zombie blood or anything like that on you. The reason that people who get bit turn is because of death through infection. It is not the zombie bite that kills them but instead is infections from the unsanitary conditions of their mouth that gets transferred to the bloodstream of the living that causes an infection that is ultimately fatal if not treated. Scratches could potentially spread the same things as they get raw meat and such under their nails and it rots some...along with their own rotting flesh.
But it is stoppable if treated quickly as we have seen with scratches becoming nothing, and Herschel getting his leg hacked off before infection could spread.