Causes

Causes of pink eye include:

• Viruses
• Bacteria
• Allergies
• A chemical splash in the eye
• A foreign object in the eye
• In newborns, a blocked tear duct

Viral and bacterial conjunctivitis

Most cases of pink eye are typically caused by adenovirus but can also be caused by herpes simplex virus, varicella-zoster virus, and various other viruses, including the virus that causes coronavirus disease 2019 (COVID-19).

Both viral and bacterial conjunctivitis can occur along with colds or symptoms of a respiratory infection, such as a sore throat. Wearing contact lenses that aren't cleaned properly or aren't your own can cause bacterial conjunctivitis.

Both types are very contagious. They are spread through direct or indirect contact with the liquid that drains from the eye of someone who's infected. One or both eyes may be affected.

Allergic conjunctivitis

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 signs and 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 sneezing and watery nasal discharge. Most allergic conjunctivitis can be controlled with allergy eyedrops.

Conjunctivitis resulting from irritation

Irritation from a chemical splash or foreign object in your eye is also associated with conjunctivitis. Sometimes flushing and cleaning the eye to rid it of the chemical or object causes redness and irritation. Signs and symptoms, which may include watery eyes and a mucous discharge, usually clear up on their own within about a day.

If initial flushing doesn't resolve the symptoms, or if the chemical is a caustic one such as lye, you need to be seen by your doctor or eye specialist as soon as possible. A chemical splash into the eye can cause permanent eye damage. Persistent symptoms could also indicate that you still have the foreign body in your eye — or possibly a scratch over the cornea or the covering of the eyeball (sclera).