When someone is diagnosed with conjunctivitis, the person may wonder what causes conjunctivitis. There are different types of conjunctivitis. The types of conjunctivitis, also called pink eye, are categorized based on what causes pink eye. Each type of pink eye has a different cause.

Bacterial conjunctivitis is caused by a bacterial infection. The surface of the eye normally contains a healthy amount of bacteria. If the amount of bacteria changes, the person becomes sensitive to the normal bacteria, or if foreign bacteria invade, the person develops bacterial conjunctivitis. All forms of pink eye generally cause a reddish, inflamed appearance of the eye. Bacterial conjunctivitis also causes a thick, yellowish discharge from the eyes. Bacterial conjunctivitis may be treated with antibiotic ointment or eye drops.

Viral conjunctivitis is caused by a viral infection. Viral infections, including viral conjunctivitis, do not respond to antibiotics. Therefore, cases of viral conjunctivitis are generally allowed to run their course. Viral conjunctivitis often subsides within two weeks. The symptoms of viral conjunctivitis include reddish or pinkish eyes, watery discharge, mild pain in the eyes, itchiness, mild sensitivity to light, and a sensation that there is something in the eye. People with viral conjunctivitis may also have symptoms of a common cold due to the viral infection.

Allergic conjunctivitis is an allergic reaction caused by exposure to allergens. Someone who is sensitive to allergens has an oversensitive immune system. The sensitivity of the immune system is inherited. People with allergic conjunctivitis often have family members who have this condition, eczema, asthma, or other allergies. Allergic conjunctivitis can be triggered by allergens in the environment such as pollen or mold. This condition can also be caused by an allergen on the eye like contact lenses, dirt on contact lenses, irritation due to damaged contact lenses, or the chemicals used to clean contact lenses.

The common symptoms of allergic conjunctivitis are pinkish eyes, watery eyes, itchiness, and slight sensitivity to light. Treatment for allergic conjunctivitis may be used to alleviate the symptoms. Over-the-counter eye drops may be comforting. Prescription eye drops for allergic conjunctivitis may contain a decongestant, antihistamine, or anti-inflammatory medication. Some people with allergic conjunctivitis may undergo allergy testing to identify the substance causing the allergic reaction.

Direct contact with chemicals or trauma to the eye can cause conjunctivitis. These types of conjunctivitis are not contagious. The doctor may prescribe treatment based on the cause in these cases. Neonatal conjunctivitis is not a separate condition based on the cause, but rather by the fact that it affects newborns. Neonatal conjunctivitis can be caused by a bacterial or viral infection or by a blockage of the tear duct.