It's generally not smart to eat meat from animals that were ill, unless perhaps you were starving. Even though most chicken respiratory diseases are viral and not transmittable to humans (with some notable but rare exceptions), they are often accompanied by internal bacterial infections which *are* transmissible. Generally animals being culled (or dying on their own) from infectious or cancerous reasons should not go to the kitchen. If a chicken meets with a sudden *accident* and you euthanize it quickly or can get it only minutes after it doed, you can genearlly eat THAT, although it may not be the highest culinary quality meat.
GOod luck, have fun,
Pat