Getting back into chickens, wary about an old condition....

Hello, I am new to this forum, and just getting back into fooling with chickens again. A long time ago I started off with 15 to 20 head, then I ended up with over 100 head, but anyway my question is, my chickens would sometimes get a condition where this yellow "growth" would begin, most of the time in or around the mouth or beak. It sort of reminds you of scrambled eggs. This would sometimes become very bad, and eventually lead to death. I wanted to get back into chickens, but I dont want to have this problem again. Does anyone know what this condition is, and why my chickens developed it? Any help or suggestions greatly appreciated.