What Causes Conjunctivitis

There are several causes of conjunctivitis, including:

  • Bacterial infections
  • Viral infections
  • Allergic reactions
  • Irritants
  • Blocked tear duct
  • Others

These are discussed below.

Viral and bacterial infections

Most cases of pink eye are caused by adenovirus but also can be caused by other viruses, including herpes simplex virus and varicella-zoster virus. Bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also cause conjunctivitis. Both viral and bacterial conjunctivitis can occur along with colds or symptoms of a respiratory infection, such as a sore throat.

Allergic reactions

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). IgE triggers special 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 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 eye drops. Allergic conjunctivitis is not contagious.


Irritation from a chemical splash or foreign object in your eye also is associated with conjunctivitis. These include chemicals, smoke, and other environmental irritants. The eye coming into contact with things that can irritate the conjunctiva, such as shampoo or chlorinated water, or a loose eyelash rubbing against the eye can also be a cause. Sometimes flushing and cleaning the eye to wash out the chemical or object causes redness and irritation.

If flushing doesn’t resolve the symptoms, or if the chemical is a caustic one such as lye, see your health care provider or eye specialist as soon as possible. A chemical splash into the eye can cause permanent eye damage. Ongoing symptoms could indicate that you still have the foreign body in your eye. Or you also could have a scratch on the cornea or the membrane covering the eyeball, called the conjunctiva.

Blocked tear duct

Blocked tear duct can also lead to an overproduction of tears, which can make the eyes more susceptible to infection. The excess tears can also create a discharge which can cause inflammation and redness. A blocked tear duct can happen due to a number of reasons, such as congenital malformations, infection, inflammation, or injury to the duct. In infants, blocked tear ducts are common and usually resolve on their own without treatment. However, in some cases, a blocked tear duct may require medical treatment, such as antibiotics or surgery to unblock the duct.


Conjunctivitis can also be caused by certain medical conditions such as blepharitis, dry eye, and uveitis.

Risk factors

Risk factors for pink eye include:

  • Exposure to someone infected with the viral or bacterial form of conjunctivitis.
  • Exposure to something you’re allergic to, for allergic conjunctivitis.
  • Using contact lenses, especially extended-wear lenses.