I would suspect that your chicken has symptoms of mycoplasma gallisepticum or MG, also called crd for chronic respiratory disease. It is one of the bacterial diseases, and you should close your flock to bird ever leaving your flock. It is spread by direct contact and in droplets in the air, and also can be passed through hatching eggs to baby chicks. Tylosin, oxytetracycline, and tiamulin (Denagard) are some of the antibiotics that will treat, but not cure it. It can come back again when they are stressed, such as molting or during severe weather. This is a good article about it:
https://extension.umd.edu/sites/ext... Mycoplasma gallisepticum (MG) Infecti....pdf
https://extension.umd.edu/sites/ext... Mycoplasma gallisepticum (MG) Infecti....pdf

