Lameness, swollen leggs. There are also some very good salves, wich will help quickly. Clean the pen throughly. Paralysis, fist making of toe's, while the leg is stretched behind or forward. It's possible that the illness comes months after infection.
Young chickens can get a vaccination.
A lot of sneezing, wet nose hole's. If a lot of chickens have this disease there's a typical smell in the pen. Green stools, total paralysis. Hanging of the head. No treatment, only preventive vaccination.