Mycoplasma Gallisepticum. it is similar to a bacteria (but not one).
you need to post more info- do you think you have this in your flock? are you trying to avoid it? are you looking for treatment options? ect....
I bought 2 hens from a chicken swap they looked fine then but now there eyes are swollen and foaming and they sound like they have a cold. I am very lucky that I keep new birds in a seperate pen on the other side of the farm.
How Should I treat these hens?