As far as I can tell, a zombie has both a yellow health bar and a red health bar. The yellow health bar indicates the amount of stamina a zombie has left; once exhausted by taking damage it collapses. The red health bar indicates the amount of health the zombie has left; once depleted, the zombie "breaks", or dies permanently.
Still, the way that depleting the red health bar works confuses me, and I want to make sure that I kill zombies as quickly as possible to keep my weapons in tip-top shape. Do I deal less health damage to a zombie while it still has stamina? Will aiming at the zombie's head inflict more health damage, even when it's knocked down? Do certain types of weapons inflict more health damage compared to stamina damage, regardless of their base damage (for example, if I have a firearm and a blunt weapon with the same attack value, will the firearm inflict more health damage)?