Sick chickens

[Henny Hen]

My chickens have all the signs of a respiratory infection. All my chickens are free ranged except for my babies that I just hatched in July. I have them in a dog pen that's fenced off. I thought that they were free of getting sick since they are on medicated feed. My husband just told me that they also are showing signs of a Respiratory infection. Now I don't know?

 

[appps]

The medicated feed is sort of like immunising against coccidiosis (but not really) I've never heard that it protects against anything else?