So it appears that if 2 zombies hit you at the same time you'll go down, which suggests that their damage is 50, however if you let 1 zombie hit you 2 times you aren't downed, you'll be left with a small amount of hp, what this may suggest is that the damage the zombie deal per hit varies, 50 on first hit (allowing for a 2 hit down w/o jugg if 2 zombies hit at the same time) and 49 or 48 on second hit, leaving you with a small amount of hp (allowing for a 3 hit down w/o jugg if only 1 zombie is present)
(Example even with jugg: 4 zombies hitting you at same time and you'll go down, it's a 4 hit down instead of 5)