We had some chicks in a brooder that started dying one right after the other. We tried saving them, but all but one of them died within a few days time. When we moved the last 3 that were living into a different brooder, my husband pulled the old one out and found a lot of mold under it on the ground. We think that may be the reason that they all died. Out of the 3 that we finally moved out, two more died that same day. We only have one left now. (It has been about a month and the one that survived is healthy now.)
I am not certain that the mold killed them, but we think that was probably it.