Pink eye is an inflammation or infection of the conjunctiva, the normally transparent membrane lining the eyelid and part of the eyeball. Also known as conjunctivitis, it may affect one or both eyes.

Pink eye is the most common type of eye infection and occurs frequently in the United States. People of any age may develop pink eye. However, preschool-age children are at higher risk for pink eye than people in other age groups.

There are several different forms of pink eye, which may be infectious (e.g., viral, bacterial) or noninfectious (e.g., allergic or the result of reaction to irritant or injury) in nature. This topic focuses mainly on the infectious forms of pink eye.

Bacterial and viral infections are often at the root of pink eye. People may become infected after coming into contact with the tears or discharges from the eyes of an infected person. People with pink eye who also have an upper respiratory infection can spread the illness through droplets that are disseminated when coughing or sneezing.

Viral infections are the chief cause of pink eye, with adenovirus being the chief culprit. This form of pink eye tends to be spread through exposure to contaminated fingers, swimming pools and medical instruments. Many patients who develop viral pink eye have an associated upper respiratory tract infection. Incidence of viral pink eye tends to be highest in the late fall and early spring.

Less commonly, bacterial infections also may cause the illness. These infections usually stem from bacteria such as Streptococcus pneumoniae and Hemophilus influenzae (which tend to affect children), or Staphylococcus aureus (which tends to affect adults). The sexually transmitted diseases chlamydia and gonorrhea also can cause pink eye in people who touch an infected person’s genitals before touching their own eye. Pregnant women who have these sexually transmitted diseases while giving birth may transmit pink eye to their newborn infants. Bacterial infections are more likely to occur in the winter and spring.

Both viral and bacterial pink eye often are associated with colds and are highly contagious. People may become infected through close contact with some who has pink eye. In addition, pink eye can be spread when people touch a contaminated object before touching their own eyes, nose or mouth. Pink eye may occur in one or both eyes. It commonly begins in one eye and spreads to the other when the patient touches or rubs their eyes.

Pink eye also can develop from noninfectious sources. Allergies also are a frequent source of pink eye. This occurs when the body has an allergic reaction to an allergen such as pollen, mold or dander. Pollen is the primary allergen that triggers allergic conjunctivitis.

Other potential causes of pink eye include foreign objects that become lodged in the eyes, exposure of the eye to a chemical or other irritant (e.g., shampoo, dirt, smog, smoke, pool chlorine). Fungi and parasitic agents also can cause pink eye. However, this is uncommon.

Children who are born with an incompletely opened tear duct may also develop pink eye. This is because tears are needed to protect the conjunctiva by diluting bacteria and washing it away. Tears also contain enzymes and antibodies that kill bacteria.