My experience with illness has basically been.......hen's not laying, pale comb, slower to move than normal but still eating and drinking. This lasted 2 days then the hen died. What was wrong with her? Don't know, didn't really investigate. Rarely, a bird just dies for no apparent reason. So, not much experience to go on.
If I had a bird with overt respiratory symptoms, I'd cull the bird. As in kill. As soon as I noticed the symptoms. I won't spend money on treating birds, or nurse unhealthy birds. If you're a chicken around here, seems you either get better on your own or you die. I've spend time and money on special needs animals in the past and am kind of over it for now.
X2
We have had some hens just... die. Overnight. No symptoms. Not all at once, mind you. Maybe one every other year. We've also had single chicks just... die. Still don't know what killed them. But, in any large group of animals, you'll have some mortality.
We very rarely have overtly sick, symptomatic hens. It's only happened three times or so, but we have a hospital/broody pen that has its own little attached run. It can be moved, so it's far from the chicken house and pasture. When I've seen a hen that's looking off-color, she went into the pen for observation. I have had one hen get better--I just think I was being over-cautious. The other two, it seemed respiratory, and they didn't look any better that evening, so they were culled. I'm with donrae, we don't mess with respiratory stuff. It's so contagious and you can lose a whole flock while trying to save one bird. We keep Tek-Trol disinfectant to spray down the hospital/broody pen and our shoes, so we don't spread disease. Tek-Trol can be found at
TSC, and it's what my father uses to clean sick pens, calving pens and obstetrical instruments (he's a dairy farmer).
You'll spend a ton of money on nursing sick birds, and you probably won't be able to find a vet to help you. Most vets, even those with agricultural practices, don't know a thing about poultry any more. We are lucky that we have a true avian specialist near us (he mostly handles parrots) but it costs me $35 just to have my bird seen. The bird is worth...$12. Sad, but true.
I do spend my money on preventative things, like vaccines for chicks and dewormer.