Pink Eye, or conjunctivitis, is a condition characterized by the infection or swelling of the membrane called the conjunctiva that covers the inside of the eyelids and the white portion of the eye. Irritation and swelling causes blood vessels in the eye to dilate, giving the eye a pink or red appearance. Pink eye is typically caused by an allergic response, infection from a virus or bacteria or irritation from foreign matter.
Allergies
Allergic conjunctivitis is a type of pink eye caused by the body's immune reaction to a certain substance. Pollen, animal dander and dust are common substances that trigger the body to release inflammatory substances called histamines, according to the Mayo Clinic. Histamines cause the eyes to become severely itchy and watery, and lead to swelling of the conjunctiva and eyelids.
Viruses and Bacteria
According to the National Institutes of Health, viruses are the most common cause of pink eye. Viral pink eye commonly accompanies other bodily infection such as the flu, measles or cold.
According to All About Vision, the most common bacteria that cause pink eye include Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumonia and Haemophilus influenzae.
In some cases, sexually transmitted bacteria and viruses may cause pink eye. For example, the herpes simplex virus Type 1 that causes sores on the mouth or face may also cause conjunctivitis. Mothers who have herpes simplex virus Type 2 may spread the virus to their infants, infecting the newborn's eyes during delivery.
Gonococcal and chlamydial conjunctivitis are other types of pink eye caused by forms of the bacteria that cause the sexually transmitted diseases gonorrhea and chlamydia. Newborn babies may also be infected by trachoma--a type of sexually-transmitted chlamydial infection that can cause scarring on the eye's surface, according to All About Vision.
Contact lens wearers--especially those wearing extended-wear lenses--may be particularly susceptible to contamination of the eye from bacteria or viruses. If the lenses are handled with unclean hands, or are not stored and cleaned properly, bacteria can grow in the lens case and on the lenses themselves. Giant papillary conjunctivitis (GPC) is an inflammation of the conjunctiva not caused by bacterial or viral infection that can occur spontaneously as a result of contact lens wear. It's likely due to the body's immune response to the lenses.
Foreign Matter and Non-infectious Causes
Pink eye can sometimes be caused by sensitivities to non-infectious substances or foreign matter irritating the eye. For example, according to All About Vision, smoke, perfumes, diesel exhaust and sensitivity to certain herbs such as eyebright and turmeric can cause some forms of conjunctivitis. Chemical splashes from household cleaning solutions or other substances can also cause inflammation of the conjunctiva, according to the Mayo Clinic.
