If the vet didn't perform any actual test, I would take his advice and diagnosis with a grain of salt.
Coccidiosis is an intestinal disease caused by parasites. Most of the time the disease progresses extremely rapidly, often resulting in death, and is highly contagious. Common symptoms do include blood in the poop. Personally, I feel it is highly unlikely at this point that your chick has coccidiosis since it has been nine days since your original post and your chick is still alive and the other chicks aren't sick. Medicated chick feed, when used, is used from the day you acquire your chicks until about eight weeks old and is used as a preventative measure to avoid coccidiosis. That is the only reason medicated chick feed is used. It is not a sure-fire way to prevent the parasite, and it not recommended that you switch to that feed at this point. Corrid is a treatment against coccidiosis. There typically not any side effects in using Corrid, so it's generally a good idea to treat for coccidiosis with Corrid when there is doubt regarding the diagnosis.
Gapeworm can be diagnosed with a fecal exam, but the adult worms live in the respiratory tract, not the digestive tract, so it is unusual if not impossible to ID the worms in the poop. What is found in the poop are the eggs, which are passed through the digestive system into the earth, waiting to be eaten by another chicken so they can hatch. Given that the vet said it is not common in Kentucky, and given the age of the chick, it is also highly unlikely if not impossible that your chick has gapeworm.
Respiratory disease is very common in chickens, but can be in a viral, bacterial, or even fungal. All forms are contagious, but obviously only the bacterial and fungal forms are treatable (in the form of antibiotics for the bacteria). There are different viral strains with different expressions and symptoms, and most of which will reside permanently in the chicken, and some chickens will live with life-long symptoms, some will succumb to the virus and die, while others may be briefly ill and suppress the virus. Sometimes steroids can help the chicken, and sometimes the viral infection can cause a secondary bacterial infection. Fungal infections are usually caused by moldy feed and/or bedding and removing the mold source is usually effective in this case.
My recommendation at this point is to contact your state's Department of Agriculture. They will be able to offer better regional advise. If you're form KY, here's a link to their site:
https://www.kyagr.com/statevet/poultry.html
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