It's a protozoa that lives in the soil...pretty much found everywhere. The birds eat it and it multiplies inside them. So....they are probably all exposed, but they might not all get sick from it. They can usually handle a small amount of cocci and then build an immunity to it.
I don't know about your pullet, though. If that's only her 2nd egg in the last month, than a small amount of blood might be "normal". But if it's cocci, she needs to be treated right away because it can be fatal. I hope someone with more knowledge sees this and can help you out....I don't really know how to diagnose it for sure, other than taking a stool sample to the vet for testing.