I have often mentioned the fact that pet parrots live with chronic stress which, in turn, depresses their immune system so this is right up our alley, as parrot caregivers. This article is about how a defective immune system creates, over time, a stronger pathogen that will affect even more negatively the next victim and, in my opinion, the reason why mortality is so high whenever you have an epidemic in a birdroom or aviary.
https://www.sciencedaily.com/releases/2 ... 144204.htm