Are you worming your chicks? I understand that Georgia may have a higher incidence...at least according to some breeders who have posted on the Heritage thread. Also, wild birds bring in about 60 varieties of diseases that our flocks respond to sometimes in low numbers and sometimes in greater numbers.
Mortality in high numbers should have a traceable cause...thus testing is extremely helpful.
Here in my area of Oklahoma, some years are harder than others. I keep an arsenal of remedies on hand but try to stick to the most simple strategy of "an ounce of prevention is worth a pound of cure". Medicated feed for chicks, worming on a schedule, spraying the remises with permethrin for flies and mites and frequent handling of each bird in the flock are necessary to maintain a watchful eye on the health of my flock.
A few years ago, I lost a third of my chick crop and 1/4 of my layer flock to a respiratory infection brought in by sparrows. Necropsy by a university vet lab on both a live sparrow and a live chicken as well as a dead sparrow and a dead chick gave me test results and a treatment strategy for the entire flock. This included both medicating and upgrading the wire cloth on all pens with young birds.to keep out the wild birds.
Of course, in my opinion, the healthiest flock is one that freeranges and babies raised by hens that freerange...exposed to the ground so they develop natural immunities to germs in their environment.