What causes conjunctivitis?

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- Infection is the most common cause.
- Allergy is another common cause. For example, many people with hay fever (allergic to pollen) have red and inflamed conjunctiva.
- Irritant conjunctivitis sometimes occurs. For example, your conjunctiva may become inflamed after getting some shampoo in your eyes. The chlorine in swimming baths is a common cause of mild irritant conjunctivitis.
The rest of this leaflet is about conjunctivitis caused by infection.

Types of infective conjunctivitis

Common infective conjunctivitis

Most cases of infective conjunctivitis are caused by common bacteria and viruses - often the same ones that cause coughs and colds. Conjunctivitis commonly develops when you have a cold or cough. Sometimes it occurs alone. In the vast majority of cases, infective conjunctivitis is not serious and clears within a few days without leaving any permanent damage to the eye.

More serious types of infective conjunctivitis

Rarely, infective conjunctivitis is more serious. For example:

- Conjunctivitis may develop in addition to keratitis (infection of the cornea). This is most commonly due to an infection with the herpes virus (the cold sore virus). If you have keratitis you are likely to get eye pain rather than just surface irritation, and often blurring of vision.
- A virus called adenovirus can sometimes cause a serious and prolonged conjunctivitis.
- Conjunctivitis in newborn babies caused by germs called chlamydia or gonorrhoea. These are serious sexually transmitted infections and need urgent treatment if they affect the eye of babies. (If a mother has one of these infections in her vagina, they can be passed onto the eye of their baby during childbirth.) Note: this is different to the very common 'sticky eye' of newborn babies caused by a blocked tear duct. A blocked tear duct with sticky eye does not cause redness and inflammation of the conjunctiva. See separate leaflet for details.
- Some adults develop conjunctivitis due to chlamydia.
- When conjunctivitis is just part of a more serious infection of deeper structures of the eye which may be indicated by pain in the eye, reduced vision, or swelling around the eye.

- Next: What are the symptoms of conjunctivitis?