I lost a chick a few days ago at age 1 week. I took her to Penn State University's Animal Pathology lab and they did a necropsy. I wondered if she had anything that might be transmissible to the other chicks. They discovered she had an infection of the yolk sac and also had omphalitis, which is where the navel opening isn't completely sealed (many times from forced hatch at a hatchery) and the bacteria gets in that way. What I am now waiting for is analysis of the bacteria- the pathologists were more concerned that she might have been infected with salmonella, which is highly transmissible to other chicks. I have seen foamy brown diarrhea with one chick and intermittent diarrhea with another. I only mention this because I was unaware of these conditions and the DVM sounded somewhat ominous if it is salmonella. I'm not sure if it is of any application to your issue but just something to keep an eye on if it starts spreading to others. Good luck!