ISA Brown only one to be infected

[Montenegro]

My ISA Brown is showing some signs of corayza, but they are the only one being affected by it.

I have 26 ISA Brown and all of them are infected, however I put 11 and 15 ISA Brown on two different farms (with different breeds to go along with them). The Black Australorp and Rhode Island Red that are on the same coop as them is not affected (on both farms) it seems it only transmits to them exclusively. What should I do with this?

 

[TheOddOneOut]

Treat all the birds.

 

[Eggcessive]

Have you had testing to know this is coryza, and not something like MG (mycoplasma gallisepticum?) What exactly are all of the symptoms? Is there a bad odor from the face?