Here is a list of diseases:
Mareks - usually affects the legs, which results in paralysis, contagious, cull the infected bird
CRD - appears when a mild respiratory infection has not been treated, can be slowed down, but is eventually fatal, highly contagious, either isolate, or cull the infected bird
Leucosis - A disease that pretty much every chicken has a little bit of, can cause blindess, dry feathers, cloudy eyes, not sure if contagious
Mycoplasma Gallisepticum - it is a a disease like CRD, however, can be cured through weeks of treatment, and causes severe breathing problems in the chicken until treated
Aspergillosis - this is caused by a fungus that grows on chicken feed, like the other diseases I have mentioned, this affects the respiratory, it fills the chicken's air sack, so that they can't breathe, and is very hard to remove
I hope this helps!