I had one die yesterday from it because we did not catch it in time. It does not take long for it to kill them...I have checked my chicks' butts daily since I've had some issues with pasty butts, and this one was fine 24 hours before it died, perfectly clean. Then the next day when I went to feed them, one was laying down and not responsive to anything...I picked him up to see what was wrong, and his rear was all covered...cleaned him off and tried everything I could but he still died a few hours later.
So the moral of the story is, check them often, and clean those butts. If you have to get the chick very wet to clean it, use a blow dryer to dry it off before letting it go again, so it does not get too chilled.
I checked my fuzzies' butts for the first week and found 2 pasted each of the first 2 days, but none after that. We cleaned their tushies by soaking the poop til it was softened and pulled it off. One person suggested to gently clip it off w/ scissors and I think it may be less stressful to do than my way.