Pale combs and respiratory difficulty could be a few things. Chicks carry prenatal immunity for about 3 weeks. After that it is up to nature and the person taking responsibility for providing proper nutrients and medication if needed. It sounds like an anemic condition. When red blood cell deficiency takes place birds get pale, lack energy, lose weight, and die. Red blood cells are vital because they transport oxygen and carbon dioxide to and from the tissues of the body. Without oxygen, there can be no breakdown of of nutrients into energy. Without energy, the chicken can't maintain normal body functions, can't utilize its feed and will die.
Red blood cells are created in the bone marrow made up of B-12, protein, and iron. Hemoglobin is comprised of protein which gives blood the red color. Hemoglobin is what retains oxygen while it is transported through the body. Since red blood cells only live about 35 days, they need to be replenished by nutrients. In the bone marrow the B-12, Iron, protein, etc., are put together to form a new red blood cell. Nature isn't perfect, and iron that doesn't make it back to bone marrow, old ones aren't returned to the spleen, then get passed in droppings. Not all feeds are formulated properly either. Thinking about this gives realization as to why we replenish diets by supplementation of nutrients.
Anemia, whether it is caused vitamin/mineral deficiency or external/internal parasites show the same symptoms. Many people don't realize that just because there is no blood in droppings does not mean coccidiosis is not the problem. From at least a month old to 9-10 months, birds should be treated for coccidiosis.
Worms also cause anemic problems since they steal vital nutrients from a chicken that create red blood cells. When chickens have access to a variety of insects, earthworms, slugs, snails, wild bird droppings it is not uncommon to worm every month.
So, without going on about this, when a bird shows anemic symptoms, treat for coccidiosis, then worms, then external parasites like lice/mites, then supplement with vitamins, minerals, and probiotics. I'm sorry some of your chickens died. We should all desire to learn more to keep our birds healthy, and no one knows everything, so we strive for more knowledge. That way we can help each other while cutting out the numerous misinformation often presented in regard to poultry. Without the balance of nutrients and required medication for specific diseases, there will only be higher incidences of mortality. People should also demand high quality feeds put out by responsible formulators and boycott the mass produced garbage.