Chickens do NOT get colds. Disinfect the whole brooder, change all the litter, disinfect the inside and outside of feeders, all perches, everything.
It's either VERY bad news or the bacteria MG.
MG you get from dirty, wet, litter or molds or other stuff accumulating. Remove the affected chick to something you can keep very dry, warm and clean. Unfortunately younger ones will succumb to it rather more quickly than older chickens. It's very common in the spring after coops have been sealed up and humid all winter.
Otherwise it's something worse and bad ...stuff... is going on.
MG is more common. And with all the rain, I'd expect it first.