Zombies have yellow eyes for two main reasons. 1. Their liver is no longer functioning to flush toxins from their body. Toxins are building up in their more liquid body cavities (including the eyeballs) as these toxins are being produced in ever greater quantities by the bacteria responsible for decay of the necrotic tissue.
2. As the zombie was turning, the pathogenic bacteria caused pus and white blood cells to build up in the eyeball's fluids. This physical reaction to the infection there is happening throughout the body but is mainly visible in the translucent tissue of the eyeball.
A corollary to this theory is that all the more fluid spaces within a zombie's body are full of pus/toxins manufactured by bacterial waste. This would go a ways toward explining why zombies are "juicy" even though they don't drink water or other fluids.
Most of the toxins would have to be endogamous...I think this is the correct term...produced by the native flora. Toxins from the consumption of brains or other people would not amass in sufficient quantities to turn the eys yellow or make the body cavities "juicy".
Zombie brains and meningeal fluid is the pathogenic fluid responsible for transmission of undead-ness.
In the mortuary industry it's referred to as "frothy purge"- the fluid that sometimes finds its way into the mouth and sinus cavities due to the decomposition of the brain matter. It tends to be dark, foamy, and highly bacteria-laden. When a corpse is resting in an uprght posture this fluid, rather than accumulating in the brainpan and being contained there, leaks down into the mouth and nose.
Zombies are decaying corpses which are walking around in an upright position. The craving of zombies for brains implicates this organ's tissue in the process of the virus. Therefore I postulate that the frothy purge of zombies is the vector which transmits their disease.
It is conceivable, since human bite wounds with no viral pathogen are notoriously prone to infection, that the rough surface of human teeth is a perfect way to introduce bacteria into a wound. However viral and pathenogenic transmissions would be difficult unless carried by saliva;bloodborne pathogens would be difficult or impossible to transmit this way unless gums were bleeding, etc. Unless, that is, another bodily fluid was present in the mouth, coating the teeth.
As I stated earlier, zombies crave human brain. Perhaps the dissolution of their brain and the mode of transmission are related to this craving.
Having read your discussion on zombies with great interest, I am left to wonder if the eyes actually decompose before the rest of the body, -as soon as the brain perhaps and other juicy cavities, but before the arms and legs? If so this might serve to explain their apparently bad eyesight; possibly what causes them to always walk with their arms outstretched before them as if to grab someones throat and gobble their brains out even when there are no living humans around?
That's a good question.
In forensics, it's acknowledged generally that the eyes are among the first soft tissues to decompose. Loose bacteria-filled blood and meningeal fluid leaks through from the cranial sac and sinuses, and fills them, after an initial period in which they sink back and deyhdrate into the skull.
Depends also on which way the body is lying, face down the eyes will fill, and face up the eyes will empty. Much like livor mortis, in which the blood pools at the lowest point in the body.