Meet Pipsqueak. He (I just get the feeling it's a he) is 12 weeks and 2 days old. He appears to have stopped growing and isn't feathering well. The bird on the right is his full sibling. (The bird on the left is 3 weeks older).
Some birds just fail to grow properly. They are sometimes just runts, or have some internal problems. Many times, the birds will grow up normally in the end, but other times, they die. I myself have had a runt chick that never grew properly and eventually died.
As long as "he" seems happy and is eating and drinking, just let him live. Make sure that he gets plenty of high-protein foods like scrambled egg, and watch out for other chickens picking on him.
Not in all strains, and there are at least 9 strains that affect poultry, IIRC. Treating for coccidiosis will not harm them if they don't have it; not treating if they do can be deadly. Here is casportpony's thread on dosing: https://www.backyardchickens.com/t/...-dosing-and-data-sheets-updated#post_11899589
I just had this show up for the first time last week, used 1.5 tsp/gallon of powder corid for 5 days. I treated all my birds, not just the sick one and as of tonight everyone is doing well.
I had a chicken once, one of six chicks- who was soooo delayed in her growth and feathering...she looked like a bad joke. She was the brunt of all sorts of sad nick names. I honestly suspected she'd been exposed to radiation at some point. As a chick, she dropped all her down, with out filling in with feathers, except wing feathers! She was bald! Then she got body feathers, but no head or tail feathers! She eventualy got covered in feathers- but nothing after her legs! I mean nothing, no tail, nadda. She looked like the back half of her had been cut off. It was the weirdest looking chicken you ever saw. She was like that until she was almost two years old, and then suddenly she grew a tail, filled out her colors and she was one of the prettiest hens I had- but it took 2 years!
Just like the story of the Ugly Duckling! Hope yours turns into a swan too! Good luck, Molly