From The Urban Dead Wiki
Digestion is a zombie skill which allows the player to gain Hit Points (HP) from bite attacks or from feeding on corpses. It is also the prerequisite skill for Infectious Bite. Like all zombie skills, Digestion costs players of all character clases 100 Experience Points (XP) to purchase.
Whenever a zombie with Digestion deals bite damage, it gains HP equal to the damage dealt, invariably 4 in both cases. In addition, zombies with Digestion can if there is a dead body on same block as the zombie; this costs 1 Action Point (AP) and restores 4 HP, directly emulating the effects of a "live" bite.
A zombie cannot successfully feed on the flesh of a reviving body—instead, if this action is attempted, the player will receive the message The body is tainted with strange chemicals. The flesh fizzes against your tongue, and you spit it out. Such failed attempts at feeding will not waste any AP, but at the same time will not gain the zombie any HP.
Likewise, zombie trying to feed on a body that has already been fed upon ten times will receive the message The body has already been picked over, and has grown cold and indigestible. This will, as with reviving corpses, not cost any AP or gain any HP.
If revivifying bodies and picked-over corpses are present, the player will one more not expend AP or gain HP, and will receive the message The only fresh meat here is tainted with strange chemicals. It fizzes against your tongue, and you spit it out.
The "ten times" count only checks from the time of the character's most recent death, and resets upon standing up—a body fed upon ten times before standing and being killed again will be available to feed upon a further ten times.
Feeding on a body will not pass on an infection, despite the link between bites and infections.