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HealthInfo Canterbury

Causes of conjunctivitis

Conjunctivitis is usually caused by a viral or bacterial infection, or by an allergy. It can also be caused by chemical irritation, or irritation from contact lenses.

Infective conjunctivitis

Conjunctivitis girlIf your conjunctivitis is caused by a viral or bacterial infection it is called infective conjunctivitis. This often happens with a common cold or cough, especially in children. But sometimes it can happen without a cough or cold.

Usually it is not serious and will clear up within a week or so without causing any permanent damage to your eye. However, sometimes it is more serious, such as if it is caused by the cold sore virus (herpes simplex), adenovirus or a sexually transmitted infection such as chlamydia.

As the name suggests, infective conjunctivitis can spread easily from person to person, but it is not usually a serious health risk if diagnosed quickly.


Conjunctivitis in newborn babies can be serious. If your baby has conjunctivitis take them to a doctor immediately.

Allergic conjunctivitis

An allergy is when your immune system over-reacts to something it has become sensitive to. Allergic conjunctivitis can be unpleasant, but complications are rare. There are three types of allergic conjunctivitis:

Seasonal conjunctivitis

This happens at the same time each year. Mostly it is caused by pollen and happens in the hay fever season. If you have seasonal conjunctivitis your symptoms will last a few weeks or months every year, and be worse in some years and better in others, depending on the pollen count. You might also have other symptoms of hay fever, such as a runny nose and sneezing.

Perennial conjunctivitis

This is allergic conjunctivitis you have all the time. It is usually caused by an allergy to house dust mites, which are tiny insect-like creatures that live in every house. They mainly live in bedrooms and mattresses. If you have perennial conjunctivitis you may also have perennial allergic rhinitis, which causes symptoms like sneezing and a runny nose. Your symptoms will tend to be worse each morning when you wake up.

Animal allergies

Contact with some animals can cause allergic conjunctivitis.

Mechanical or toxic conjunctivitis

This is conjunctivitis caused by contact lenses, or irritants such as shampoo or the chlorine in swimming pools. There are two types of mechanical or toxic conjunctivitis:

Contact conjunctivitis

Some people become sensitised to the chemicals in cosmetics, swimming pool water, or other substances their eyes come into contact with. This causes an allergic reaction like allergic conjunctivitis. The skin on their eyelids might also become inflamed.

Giant papillary conjunctivitis

This isn't common, and we don't know exactly what causes it. It happens in some people who have a contact lens or other small object in their eye. It affects around 1% of contact lens wearers. It also sometimes happens after surgery.

These two forms of conjunctivitis can occasionally cause inflammation and ulceration of the cornea, called keratitis. If they aren't treated they can cause some vision loss.

On the next page: Symptoms of conjunctivitis

Written by Canterbury optometrists. Adapted by HealthInfo clinical advisers. Endorsed by clinical director, Ophthalmology, Canterbury DHB. Last reviewed November 2019.


See also:

Hay fever

Page reference: 142402

Review key: HICJC-49685