We are assuming that the bird that started the conversation was in fact killed by cocci. I agree that may be most likely, but you can't positively tell without cutting the intestines open. Often a bird that is burdened by cocci can be killed by a co infection. Other gut diseases can cause similar symptoms. There is somewhere around a dozen species of cocci that can infect chickens. A few are the most significant.
Chickens can develop resistance if they are exposed at low levels. High levels exist where many birds have been concentrated over a period of time. Warm damp conditions can create conditions for a population explosion, overwhelming even adult birds that have been exposed prior. These same conditions can cause other pathogens to thrive, so often the birds are burdened by more than one thing.
Adult birds that have never been exposed to it, and are suddenly exposed to higher numbers can succumb to cocci.
Chicks are especially vulnerable because there is no acquired immunity, they are concentrated in a warm space, and possibly a humid environment. The conditions could be ripe for a population explosion.
Management is the best tool against cocci. Generally speaking adults that have been exposed to it before are fine if the bedding and grounds is kept in good shape.
Chicks that are provided clean bedding, in clean brooders, the bedding is well maintained, the humidity kept low, and are not overcrowded, are not as likely to suffer an outbreak. The best possible management does not eliminate the possibility so many use medicated starter.
Cocci can become drug resistant, so management is the best first option. I will use medicated starter when I need to. I assume at some point that I will.
You are most likely to have a problem when introducing new birds that have never been exposed to what you have on your yard, the birds have weak immune systems, or management is poor.
I'll add to that...the walls of intestines are complex and are not just a blank slate where you will either get coccidia or will not according to the numbers present in the animal's environment. There are other microorganisms at work that can help inhibit an overgrowth of coccidia merely be having a stronger colonization in the walls of the intestine and their metabolism actually secretes chemicals that inhibit the reproduction of coccidia, therefor preventing an overgrowth of the protozoa internally. Keeping a balance in those bowel bacteria is key to it all, for if you have balance internally your birds can withstand some levels of external imbalance. A healthy colonized bowel is a big part of a bird's immune system that many overlook and can protect them from so many things. What few realize is that this healthy culture also produces proteins and B complex vitamins, increases absorption of key nutrients and even discourages overgrowth of parasites by making the bowel an inhospitable pH for their existence.
To have complete protection, one must have balance inside and outside with good cultures in both areas. Overstocked soils that are overloaded with nitrogen and are impacted and cannot be cleansed by the rains are hard to establish with a healthy culture and insect life that could inhibit the overgrowth of coccidia. It's not just damp and steamy environments that produce this overgrowth~though they are more prone~ but an imbalance in the soil life itself by poor drainage, over stocking, compact soils and never any attempts to correct the condition of these soils. They are just a petri dish for the wrong kinds of organisms to take hold and thrive, but are not ideal for the healthy ones that can keep the bad ones in check.
This is why some have taken a more proactive approach to livestock feeding by introducing some prebiotic and probiotic cultures into their intestines...and the commercial feed companies are also adding these things now as well.
Among the huge list of type of bacillus found in lactobacillus cultures, one in particular grabs my interest and is applicable here~Pediococcus acidilactici
Pediococcus acidilactici can function as immune modulators. Animals fed with P. acidilactici have shown enhanced immune responses against infectious coccidioidal diseases.
Pediococcus acidilactici is also known to prevent colonization of the small intestine by pathogens like Shigella, Salmonella, Clostridium difficile and Escherichia coli among small animals.
Pediococcus acidilactici has not been stated in any literature to have toxic effects. Another potential benefit of using them as Probiotics is their use as alternative medicines against infectious parasitic pathogens like Eimeria* in broiler-chicken [6].
in livestock and wild animals, infects mainly the cells of the digestive tract, although it also attacks cells of the liver and the bile duct. Symptoms of infection are diarrhea, weight loss, and general weakness. Eimeria is characterized by spore cases that contain four spores, each with two infective sporozoites. Among the common pathogenic species are E. necatrix and E. tenella (in poultry); E. stiedae (in rabbits); and E. bovis, E. ellipsoidalis, and E. zuernii (in cattle).
A healthy intestine is good for animal and human alike and is the very basic and most urgent place to start if one is going to improve overall flock health and protect them from disease transmission. Using meds is like putting a band aid on a wound that may occur again the next time one is near the dangerous object. It's best to build a guard against a dangerous object so that one never gets that wound in the first place.