Quail chick - cause of death?

[alpinewelsummer]

Hey all, sad situation here. We had a quail chick--the runt of the litter, much smaller than the others--go way downhill in 24 hours. She started off very active today but I noticed she kept picking at her booty, I checked and she was crusted up and still had an umbilical "stump" left, so I gently washed her and blow dried her, then put her in a separate thing near the heat but still where she could safely get away from it.

But eventually she just got worse, and worse, and a few moments ago she passed. She was gasping for air and I noticed before she went that her vent leaked some brownish/greyish foul-smelling fluid. I thought that maybe she had just been crusty too long as it took me a day to notice, and maybe I had set the heat too low or high or something, but the rest of her siblings are fine, much bigger than her, and crust-free. Could the rinse have chilled her too much? I couldn't get her COMPLETELY dry as she was terrified of the blow dryer and I didn't want to hurt or stress her out, so she still had a few damp patches when I put her back in her little cage.

What were the potential causes for this little one? Should I be worried for the others? Thank you all.

 

[alpinewelsummer]

I disinfected their cage and added some vitamins+electrolytes and a bit of nutridrench to their water, but I'm still wondering about this one.