I had to make an account just to rectify this blatant fallacy.
The value you want is 0, not 1.
The value is coefficient. 1 means that all damage is unmodified, the default for Easy is 2, meaning you deal 2x damage per shot.
1x damage in each category is completely wrong.
Values are as such, choose as you will
0 - Unkillable without headshot
.01 - Largely unkillable without headshot, but not entirely
.1 - Not entirely unkillable without headshot, but preserves a feeling that the zombies are very difficult to put down without
Common infected have 50 hp ("100" in expert due to .5 coefficient, 67 advanced for its value also).
Divide this 50 by the value you choose to determine their effective HP.
50 / 0 = undefined/infinite
50 / .01 = 5000
50 / .1 = 500
Compare your chosen value to the weapons here
http://woodio.net/gaming/8-pc/29-lef...age-guide.html for an idea of how many shots it'll take without a headshot.
(Short reference for point blank/long range)
Pistol - 36 / 13
Uzi - 20 / 11
AR15 - 33 / 29
Shotgun - 25 x 10 / 7 x 10
AutoSG - 25 x 12 / 7 x 12
AR15 can output 30*50 damage (simple average) or 1500 damage in one clip.
At .01, it'll take almost 3 clips to kill a single infected without headshot, at .1 you can kill 3 infected with one clip, or whatever in between .1 and .01.
So you can opt for complete immunity or just "extremely resistent".
The Hunting Rifle is the oddball, as it does 90 damage at all ranges, but can still kill a 100 HP infected in 1 shot, so I'm not sure if it simply "always headshots" or not.