Viruses, bacteria or allergies can cause conjunctivitis, but viruses are the most common cause. Newborns also may experience conjunctivitis as the result of an incompletely opened tear duct.

Viral and bacterial conjunctivitis may affect one or both eyes. Viral conjunctivitis usually produces a watery or mucous discharge. Bacterial conjunctivitis often produces a thicker, yellow-green discharge and may be associated with a respiratory infection or with a sore throat. Both viral and bacterial conjunctivitis are associated with colds. Both viral and bacterial types are very contagious. These types of conjunctivitis are common in children, though adults can be affected as well.

Allergic conjunctivitis affects both eyes and is a response to an allergy-causing substance such as pollen. In response to allergens, your body produces an antibody called immunoglobulin E (IgE). This antibody triggers special cells called mast cells in the mucous lining of your eyes and airways to release inflammatory substances, including histamines. Your body's release of histamine can produce a number of allergy symptoms, including red or pink eyes. If you have allergic conjunctivitis, you may experience intense itching, tearing and inflammation of the eyes — as well as itching, sneezing and watery discharge from your nose. You may also experience swelling of the membrane (conjunctiva) that lines your eyelids and part of your eyeballs, resulting in what may look like clear blisters on the whites of your eyes.