Back to the original question of the thread...
It is more likely something internally wrong from the beginning, or an injury that you did not see. I say this because you stated the chick was just fine TWO HOURS before you found it dead, which makes me lean more towards an injury. Maybe it fell over and his neck was stepped on in just the right spot to break it? Or maybe another chick pecked a random spot on it, and broke his neck. Or perhaps another chick pecked his belly and caused an internal injury.
Whatever the cause, Cocci USUALLY takes longer to kill the chick - and you'll almost definitely see signs of illness just two hours before the chick dies. He'll at least have one of the symptoms - not eating or drinking, lethargic, white fluid oozing from the mouth when you pick it up, or it is fluffed up.
Now that we have covered the REAL possible cause of death, here's a wide view on cocci and "coccidiosis" infections.
Cocci is "found in almost every chicken coop". Just like salmonella is found in almost every chicken, the cold virus is found in almost every human, and worms are found in every dog pen. Whether or not your chicken gets the cocci into their system isn't really a question. They WILL. The question is whether or not your chicken is immune to it.
Cocci is a parasitic protozoan, not a virus or bacteria. And just like a normal adult human might be immune to the cold virus, a normal adult chicken might be immune to the cocci protozoa. The key is BUILDING that immunity. This is what "medicated feed" is all about. It keeps the number of Cocci protozoa in their body LOW ENOUGH to prevent excessive damage before the chicken's immune system learns how to fight it.
Think of it as "treating flu symptoms". We have no CURE for the flu. We have vaccines to teach our bodies how to deal with it, but no actual cure. And we only get SICK with the flu when a new strain comes along that our body is not familiar with, and our immune system does not know how to fight against. The flu virus will always be in our bodies. We only get ill when there are too many for our immune system to fight. And that only happens when our immune system does not know HOW to fight it. Once the immune system "learns" how to fight it, then our bodies have to take the time to produce the antibodies and white blood cells to fight against it. During this time, we are "ill" and experience the symptoms. And the elderly, young children, and those with weak immune systems are most susceptible of dying from it as the flu virus replicates faster than their bodies can learn. But all of the medications out there do nothing more than treat SYMPTOMS, as it is up to our body to actually kill the virus.
But even after the symptoms are gone, the flu virus stays in our body. Our immune systems are constantly fighting them.
A cocci infection is simply a matter of too many protozoa being in the chicken's body and doing too much damage, faster than their immune system can keep up - because the immune system doesn't know how to fight it yet. And sure enough, young chicks are most susceptible because they don't have immunity against ANY strain of cocci (so they may actually become infected with more than one strain at a time) and their bodies are already busy learning to fight every other germ it comes into contact with, plus the lack of development in the immune system slows down it's response to the protozoan attack. The combination on a young chick can be deadly.
This is also why SOME chicks survive and others do not. Some chicks manage to squeak by and their immune system picks up on it immediately and starts to combat the germ right away. Others may not be so quick and they may come down with symptoms. The cocci protozoa attacks the digestive system, basically eating it away bit by bit. From a bleeding esophagus, to bleeding stomach, to a bleeding intestinal tract - it'll cause it all. The clear liquid that often comes out of the chick's mouth when you pick it up, is stomach fluid. The bloody poop is simply from this internal bleeding.
Medicated feed and amprolium will both kill or slow down some strains of the cocci protozoa. But they often do not work fast enough to kill every protozoa in the chicken's body. Amprolium is stronger than medicated feed, and will help treat cocci "infections" in it's more advanced stages because of that. But regardless of which medication you use or have used, THE CHICK'S IMMUNE SYSTEM HAS TO TAKE OVER AND ACTUALLY LEARN TO FIGHT THE PROTOZOA. That is the ONLY way you can actually STOP the infection. If the medicated feed gives it the "edge" it needs to do this, then yes it will actually stop or prevent an infection of cocci in young chicks. But if it's just not strong enough, and/or the chick's immune system is weak enough, the cocci infection will continue to take hold anyway. At that point you can try amprolium to give it a stronger "edge" to fight the germ. But here again, if a chick has a birth defect of a very weak or "deformed" immune system, even Amprolium will not help. ANY MEDICATION you use, regardless of strength, is ONLY an agent to HELP the chick until his immune system can take over. PERIOD.
The problem with both of these "treatments" is that once the chick pecks anything on the ground where he originally got the germ... he has the protozoa back in his system all over again. HOPEFULLY by then, he has gained an immunity to it. If not, the chicken can still get a cocci infection, even if he has had it before. Furthermore, every time a chicken poops, and he has the protozoa in his body, he can and often will spread it to the others in the flock, including back to himself.
So really, I can not stress enough that the chicken's immune system is the ONLY actual "cure" against the cocci protozoa. Medications of ANY sort merely help keep it at bay while his body learns to fight it. This is why medicated feed "does not prevent" an infection. Because if you really think about it, every chicken out there really already has this infection. And they probably always will.
(By the way, infections that "wipe out a flock" come from new strains that the flock has never been in contact with, and is not immune to - similar to new strains of the flu virus, such as the old H1N1 Swine Flu that caused so much havoc in the world recently.)