My chickens are sick

I am new to both this site, and to chicken ownership, but I have found this site so incredibly helpful. Last night, we lost a 7 week old chick we had raised like our baby. She appeared to have a seizure- it flopped onto its back, and began shaking violently, then went still. My husband witnessed it, and ran to the house to get me. I grabbed her, and she eventually perked up a little. I got her to eat and drink a little, and eventually she could walk. I thought she was on the upswing, but she passed during the night. We are heartbroken. (I should mention that she did have a birth defect-her beak was not straight, but off to the side of her face.)
We have a red comet that acts very similarly, but without the seizure. We rescued her from a chicken farm, where she was being abused by other chickens.
Today, one of our adult leghorns came down with similar issues. Her comb is drooping, she's gasping, closing her eyes and shaking. We are terrified.
I know that this is a place that I can come and not be judged for how upset we are. These are our pets, and we are devastated. What is happening?