Cocci is caused by a parasitic protozoa in the soil. There are multiple kinds of it. All birds have some of these in their gut similar to how we have E. coli in ours. Their immune systems keep the numbers in check, unless/until the bird is overly stressed or comes in contact with an unknown variety. Then you may see bloody diarrhea which is the most obvious sign. Add Corid to the drinking water (lots of instructions on BYC). Corid starves the Cocci of Vit B by providing a fake Vit B, which gives the birds time to build up an immunity again.