Achievement name % of players
DRAG AND DROP
Rescue a Survivor from a Smoker's tongue before he takes damage. 73.4%
BRAIN SALAD
Make 100 headshot kills. 70.3%
TONGUE TWISTER
Kill a Smoker who has grabbed you with his tongue. 70.1%
BLIND LUCK
You or another Survivor take no damage after being vomited on by a Boomer. 68.3%
PYROTECHNICIAN
Blow up 20 Infected in a single explosion. 64.4%
MY BODYGUARD
Protect any Survivor from an attacking Infected 50 times. 63%
TANKBUSTERS
Kill a Tank without it dealing any damage to a Survivor. 60.6%
NO SMOKING SECTION
Kill 10 Smokers as they are pulling helpless Survivors. 59.9%
HERO CLOSET
Rescue a Survivor trapped in a closet. 57%
HUNTER PUNTER
Shove a Hunter off of a pinned and helpless Survivor. 56.8%
101 CREMATIONS
Set 101 Infected on fire. 54.1%
TOWERING INFERNO
Light a Tank with a Molotov. 52.2%
OUTBREAK
Catch a rare strain of infection, then pass it on to someone else. 50.7%
WITCH HUNTER
Kill a Witch without any Survivor taking damage from her. 46.6%
GROUND COVER
Save another Survivor from a Special Infected while on the ground. 43.9%
SPINAL TAP
Kill an Infected with a single blow from behind. 42.8%
DEAD STOP
Punch a Hunter as he is pouncing. 41%
BURN THE WITCH
Light a Witch with a Molotov. 40.4%
NO-ONE LEFT BEHIND
Beat a campaign with all 4 Survivors. 39%
DOUBLE JUMP
Pounce two different Survivors on one life as a Hunter. 34.3%
JUMP SHOT
Headshot a Hunter while he's leaping. 34.3%
MERCY KILLER
Survive the No Mercy campaign. 32.1%
TOLL COLLECTOR
Survive the Death Toll campaign. 29.4%
DEAD BARON
Survive the Dead Air campaign. 27.6%
CLEAN KILL
Shove a Boomer and then kill him without him splashing on anyone. 26.4%
CHAIN SMOKER
Constrict two Survivors on one life as a Smoker. 26.3%
DEAD GIVEAWAY
Heal a fellow Survivor when your own health is below 10. 26.1%
GRIM REAPER
Survive the Blood Harvest campaign. 22%
HELPING HAND
Revive 50 incapacitated Survivors. 20.2%
BACK 2 HELP
Leave a safe room to save an incapped teammate and bring them back safely. 18.4%
PHARM-ASSIST
Give pain pills to 10 Survivors. 18.3%
BIG DRAG
Drag a Survivor 100 feet with your tongue as a Smoker. 18.2%
DEAD WRECKENING
Dole out 5000 total Survivor damage as a Special Infected. 15.1%
STAND TALL
Survive a campaign without being incapacitated.C 13.8%
FIELD MEDIC
Heal 25 Survivors with a first aid kit. 12.3%
BARF BAGGED
Cover four Survivors with Boomer bile at once. 9.1%
All 4 Dead
Kill all four Survivors on one life as a Tank. 6.4%
DO NOT DISTURB
Sneak past all Witches in a campaign without disturbing one. 6.1%
AKIMBO ASSASSIN
Survive an entire campaign using only pistols. 5.7%
CR0WND
Kill a Witch with a single headshot. 4.1%
ZOMBICIDAL MANIAC
Survive any campaign on Expert. 2.8%
MAN VS TANK
Single-handedly kill a Tank. 2.5%
UNBREAKABLE
Finish a campaign without ever using a first aid kit on yourself. 2.4%
UNTOUCHABLES
No Survivors take damage after contacting the rescue vehicle. 0.9%
STOMACH UPSET
All Survivors complete a campaign without being vomited on. 0.8%
RED MIST
Kill 1000 Infected with a mounted machine gun. 0.7%
SAFETY FIRST
Play an entire campaign with no Survivors taking friendly fire damage. 0.6%
WHAT ARE YOU TRYING TO PROVE?
Survive all campaigns on Expert. 0.4%
NOTHING SPECIAL
Survive a campaign with no Survivors taking damage from Special Infected. 0.3%
ZOMBIE GENOCIDEST
Kill 53,595 Infected. 0.1%
LAMB 2 SLAUGHTER
As an Infected, incap a Survivor who has entered and left a safe room. 0%
http://www.steampowered.com/status/l4d/
I knew that would be hard, but I didn't realize that 0% of people have done it (well, 0-0.049 given the rounding)
Also it's interesting to note that 50% of people have caught the viral achievement, presumably started by Valve employees, and spread through boomer bile.
