My chickens seem to be unable to gain weight. When I lift them up their breast bone protrudes, no meat on either side. They are 4-9 months old. Many 8+ have died from something spreading in the flock or malnourishment. When they show symptoms of sickness they decrease their eating tremendously. I feed them a mixture: purina crumbles & pellets, organic scratch, meal worms, vit/mineral supplement, & grit. Water readily available. One bird had necropsy & ruled bird died of Pox. Is pox a slow deadly disease? & am I feeding my birds a sufficient diet?