I'm nowhere near as experienced as some of the old timers on here, but let's see...from what I have seen on all these forums, I don't think there is a sicker animal, than a chicken. Mites, lice, Mareks, Newcastle, Coccidiosis, fowl pox, mycoplasmosis, avian influenza, samonella, bound egg.... then there is the quarateen
I've had mites once (it was bad but once I treated it was completely under control). I've had coccidiosis once. A few cases of bumblefoot. Wry neck once (but that particular bird had all sorts of congenital issues, and even then she lived over 2 years and even laid).
Salmonella is thankfully NOT an issue in my flock and I can use my eggs raw or undercooked without worry. Mmm caesar dressing!
Hardest part of chicken keeping is that realistically there isn't the option of veterinary care for many of us. And that means learning to do surgery or put birds down yourself.