Main Causes of Respiratory Distress in Young Chickens:

Brooder Pneumonia: gasping for air, rapid breathing, teary, red eyes, drowsiness or depression are typical tell tale signs of pneumonia in chicks.
If you suspect your chicks have contracted brooder pneumonia , best thing you can do if it's within your reach is to is speak to a veterinarian.  We consult with the Chicken Doctor for all matters pertaining to the health of our birds. Peter Brown has always been kind, steadfast, and extremely reasonable. As an alternative, you can do your own research on the causes and treatment of this condition. There are many Backyard Chicken support groups on Facebook that you can join and ask questions and get the support you need. You also get to know other fellow chicken keepers that way!

Gapeworms: gasping for air with outstretched necks.

Diarrhea: Cocci is most likely the chief perpetrator  until proven otherwise by lab tests. IF YOU SEE RUNNING DROPPINGS OR BLOOD IN THE STOOL, START TREATMENT WITH CORID IMMEDIATELY!! Without prompt treatment, affected birds will die. Recommended dosage is 1-2 teaspoons per gallon of water (2 teaspoons if you have death) for 5 days. Additionally I 
personally recommend raw milk. I just recently learned for myself how raw milk was for my broilers who had started dropping overnight (this will be another article!) but I truly believe it was what saved the remaining ones from whatever it is they had. It is said that raw milk coats the stomach and relieves the pain, while providing due nourishment.

Coccidiosis - AKA Cocci - Affects chicks from 3-5 weeks. It is very common during warm weather.

Roundworms - Stagnant weight gain. The chick is eating and drinking normally but not gaining any weight. Usually affects birds during the first 3 months of life.

Necrotic enterites - Depression, sudden death. Usually takes place between weeks 2-5 and is more common during warm weather.

Infectious bursal disease -  Watery diarrhea, inflamed vents, and staggering typically affects bird's age of 3-6 weeks.

Main Types of Neurological Illness in Young Chickens

Marek's disease - Affects flocks worldwide as early as 7 weeks of age. Common symptoms include lameness, twisted neck, droopy wings, paralysis with one leg forward and one back. There is no cure for Marek's disease, but vaccine is readily available. We follow a strict Mareks vaccine program at out farm. Extremely high mortality.

Avian encephalomyeritis (AE) typically affects birds between 2 -16 weeks of age. Tell tale signs include trembling of the head, paralysis of both legs to one side. High mortality of 25 -60% .

Vitamin E deficiency, also known as "crazy chick disease." It affects birds from age 2-4 weeks. Classic symptoms shown is chick is unable to walk, fall on its back or stands with its head between its legs; head may also twist sideways or over its back. 

Vitamin B 12 deficiency: Similar to previous, the chick of less than week old may display symptom of not being able to stand and drawing its head back into a star-gazing position.