There are people out there that want eggs and meat from chickens that have never been fed non organic feed of feed with Soy or Corn. The egg farmer I am working with is setting up a pen like that for sales to folks on a special diet. I have not found medicated feed that works for that.
Some places have worse fungus that causes Cocci too and the medicated feed does not protect them long enough to build resistance.
I have a friend who has a big farm, and tries to raise everything with out feeding medicated feeds. Cattle, sheep, goats, and chickens ( Orps). She has tremendous losses every year from Coccidia, and worms. When she has outbreaks, she then medicates, but the losses and stunting are very apparent. These animals are not overcrowded either, and are all well fed .I went out to help her select some breeder birds for her to keep. I could SMELL the coccidia in the first 3, 8 month old pullets I picked up.What I was smelling, for those who do not know, was the intestinal lining being sloughed after dying, and rotting in the birds. She told me that she had just lost 3, 8 month old birds. I showed her that the 3, I had handled also were passing blood. Time to medicate....She lost a total of 10 birds before the Corid worked.
Coccidiosis is an opportunistic disease. It is caused by a protozoa , that can multiply like " The Andromeda Strain", overnight, when bird/ animals are stressed, particularly.Fungus has nothing to do with it's growth, that I know of. Over time though, a property can become contaminated with a heavy load of oocysts ( eggs ) that are passed by animals/ birds harboring them.
For this reason, I have always quarantined and medicated new birds coming onto my property. I run fecals before they enter the flock. Young birds are kept on medicated feed until after they have been integrated into the flock, by being fed in a creep feeder, to also lower stress. This works for me.