On average, how much damage does a hit from a zombie do? Are there any status effects that change how much the actual hit does?
From the DayZDB:
Zombies do little damage on their own but often cause status effects like Bleeding, Pain or Broken Bones and occassionally Infection.
Zombies can knock you Unconscious in one hit if you're under 9000 blood or if the hit took you under 9000 blood
Zombie swarms attacking players in the building may very rarely destroy the building
Zombies can hit you (and you can hit them with a melee weapon) when they may appear to be too far away
In my experience, a single hit from a single zombie can cause anywhere between 50 and 100 damage. Should this damage knock you unconscious, there is a high likelihood the zombie will continue to eat you. This does a considerably higher amount of damage; ranging anywhere from 200 - 600 every one to two seconds.
There are no status effects that will actively increase the damage a single hit will do. There are, however, status effects that will cause you to continually take damage after the hit has landed.
The very obvious one, Bleeding, will cause you to lose a large amount of blood in a very short period of time.
The second one, Infection, can very rarely occur when being hit by a zombie. Infection causes you to lose blood at a rate of 3 blood per second until you a) take Antibiotics (which are extremely rare), or 2) reach 6000 blood. Once you reach 6000 blood, you will lose blood at a slower rate.
I suppose it is technically possible for a lone zombie to kill you in a single hit, given the right circumstances. But... You'd have to be pretty unlucky for that to happen.
Any information not in block-quotes comes from my own personal experience as I roam the wastes of Chernarus