Good morning, I had a chick doing something similar a few weeks ago. I contacted the hatchery, and this is the information that they gave me. I am NOT an expert, and just a beginner, but after asking a lot of questions the person at the hatchery told me that the medicine she suggested would not hurt the chick, it could only help. Unfortunately I didn't get the chance to try it, because my chick was sick over the Memorial Day weekend and by the time the responded it was too late.
From the hatchery:
" Did she have any runny/bloody poop or pasty bottom? It sounds like it could have been coccidiousis but I am not 100% sure about that. If it was that it is not contagious to the other chickens however, stress does affect chickens at different rates so keep an eye on them and if you start to see any signs of runny poop or weekness you will need to start medicine. You can use Sulfadimethozine or Sulmet. Both of these can be found at your local feed/farm store usually in the cattle isle."
From myself:
"She didn't have abnormal poop, and her bottom was completely clean. She also didn't seem to have respiratory issues. She seemed clumsy when we got her, and then she became weaker and weaker. It became more apparent as the other chicks became stronger. I am really hoping that whatever was wrong is not contagious. The other chicks have looked strong and healthy with clean bottoms. Are there any other conditions that it could possibly be so that I can be on the look out for? The medicine above is only used for coccidiousis, and coccidiousis is indicated by runny, bloody poop?"
Hatchery:
"No that medicine treats coccidiousis, pasty bottom and several other issues as well. The runny poop is just 1 indication of coccidiousis. It really sounds to me that is what she had. All chickens have this but it don't become active in some like it does others. There is many things that can make it become active. I always explain it like this: We as humans all have germs in our body. Sometimes those germs become active and make us sick and sometimes they do not. The chickens are the same way. I don't think you will have anything to worry about with your other chickens. If they were going to get sick you would most likely see signs of it already but if they do start to get weak just start the medicine and it will clear up. Have a great day!"
I hope that this helps you, and that your chicken begins to thrive!