Unfortunately chickens dont get colds, rather the contact a variety of respiratory diseases many of which are chronic. These chronic respiratory diseases (CRD) can be bacterial, viral or fungal in nature. Although there are different treatment options even fully recovered chickens can remain infectious for life and often relapse when under stress.
Here are many of the typical forms of CRD and some other respritory related diseases with a bit of info on each which I pulled together from different sources so what you find may vary as I found some dissagreement between sources.
Coryza Cause: gram-negative bacterial Hemophilus paragallinarum (A,B,C types)
Symptoms: Cold-like symptoms; coughing and sneezing, rotten smelling nasal discharge, swelling of the face, watering or bubbling of the eyes, lethargy, reduced food and water intake.
Transmission: rapid 1-3 days by direct contact (aerosol, feeders/waters). Onset is rapid 2-3 days. Infected birds remain potential carriers for life.
Treatment: Antibiotics such as: sulfadimethoxine, sulfathiazole, erythromycin and Flouroquinolones
Notes: often coexists with mycoplasma infections
Mycoplasma (MG)Cause: Mycoplasma gallisepticum
Symptoms: Cold-like symptoms; coughing and sneezing, nose or eye discharge, swollen sinus, rails, infected air sacks, slow or stunted growth.
Transmission: Moderate to slow spread 1-2 weeks by direct contact (aerosol, feeders/waters)
Treatment: Antibiotics such as: Flouroquinolones, tetracyclines
Notes: Can spread through egg and between other types of birds, Vaccine available though not 100% effective. Infected birds remain potential carriers for life.
Infectious Bronchitis (IB)Cause: Variants of a corona virus
Symptoms: Cold-like symptoms; coughing and sneezing, possible rails, drop in egg laying quantity and/or quality, depression, huddling, loss appitite.
Transmission: by direct contact (aerosol, feeders/waters). Disease has long course typically several weeks or more. Typicallyinfects significant portion of floc with significant mortality.
Treatment: Sodium salicylate, antibiotics for secondary bacterial infections
Notes: Virus can live outside host for ~1 month. Chickens remain infectious up to 1 year post recovery.
Infectious Laryngotracheitis (ILT)Cause: Variant Herpes virus
Symptoms: Gasping, difficulty breathing, coughing mucus or blood, eye or nose discharge, drop in egg production,
Transmission: by direct or indirect contact, fairly slow spread. Often affect most of a flock with significant mortality. Course of disease can be several weeks.
Treatment: None, antibiotics for secondary bacterial infections only
Notes: Virus does not survive well outside host. Chickens may remain infectious for over 1 year post recovery.
Notes: Vaccine available
Avian pneumovirus (Swollen Head Syndrome)Cause: RNA virus
Symptoms: Cough, rales, sneezing, nasal discharge, conjunctivitis, and swelling of the sinuses.
Transmission: by direct or indirect contact as well as airborne.
Treatment: None, antibiotics for secondary bacterial infections only
Notes: Vaccine available, mostly does not affect chickens in North America
Newcastle (NC) Cause: Rubela virus
Symptoms: gasping, coughing, nervous signs (depression, drooping wings, twisting of head and neck, circling, movement disorders and paralysis, swelling of the tissues around the eyes and neck, greenish, watery diarrhea, reduced egg production and quality
Transmission: by direct or indirect contact. Incubation 4-6 days.
Treatment: None
Notes: Vaccine available
Avian Influenza Cause: Orthomyxo virus
Symptoms: Sudden death, loss of appetite, reduced feed consumption, drop in egg production, depression, coughing, nose and eye discharge, swollen face , cyanosis of comb/wattles. Diarrhea, paralysis.
Transmission: by direct or indirect contact. Incubation 3-5 days.
Treatment: None, flock destruction,
Notes: Requires emergency disease control measures.
CRD is not the only thing that can cause respiratory problems though, Other possible sources include excessive dust, mold or fungal infections, excessive humidity/ammonia due to insufficient ventilation and gape worm.