Conjunctivitis is swelling (inflammation) or infection of the membrane lining the eyelids (conjunctiva).

The conjunctiva is exposed to bacteria and other irritants. Tears help protect the conjunctiva by washing away bacteria. Tears also contain enzymes and antibodies that kill bacteria.

There are many causes of conjunctivitis. Viruses are the most common cause. Other causes include:

• Allergies (allergic conjunctivitis)
• Bacteria
• Certain diseases
• Chemical exposure
• Chlamydia
• Fungi
• Parasites (rarely)
• Use of contact lenses (especially extended-wear lenses)

"Pink eye" refers to a viral infection of the conjunctiva. These infections are especially contagious among children.

Newborns can be infected by bacteria in the birth canal. This condition is called ophthalmia neonatorum, and it must be treated immediately to preserve eyesight.

See also:

• Allergic conjunctivitis
• Keratoconjunctivitis sicca
• Neonatal conjunctivitis
• Trachoma
• Vernal conjunctivitis