Conjunctivitis is a condition that occurs when the conjunctiva (a thin layer of cells covering the front of your eyes) becomes inflamed.
The three most common causes of this inflammation are:
- infection (infective conjunctivitis)
- an allergic reaction (allergic conjunctivitis)
- something irritating the conjunctiva, such as a loose eyelash (irritant conjunctivitis)
These are discussed in more detail below.
Eye infections are most commonly caused by:
Viral conjunctivitis causes a watery discharge, while the discharge from bacterial conjunctivitis contains pus. An eye swab can also determine the cause of the infection (read more about diagnosing conjunctivitis).
Spreading the infection
You're more likely to develop infective conjunctivitis if you've been in close contact with someone who's already infected with it.
It's therefore very important to wash your hands thoroughly after coming into contact with anyone who has infective conjunctivitis. You should also avoid sharing pillows or towels with anyone with the infection.
You may be more at risk of getting infective conjunctivitis if:
- you're old or young – it's more common in children and the elderly, possibly because children come into contact with more infections at school, and elderly people may have a weaker immune system
- you've recently had an upper respiratory tract infection – such as a cold.
- you have diabetes or another condition that weakens your immune system – as you may be more vulnerable to infections
- you're taking corticosteroids (steroids) – which can weaken your immune system
- you have blepharitis (inflammation of the rims of the eyelids) – which can be caused by a bacterial infection and may lead to conjunctivitis
- you've been in a crowded place – such as a busy train
Allergic conjunctivitis is caused when your eyes come into contact with an allergen (a particular substance that causes your immune system to react abnormally). This is known as an allergic reaction.
There are four main types of allergic conjunctivitis:
- seasonal allergic conjunctivitis
- perennial allergic conjunctivitis
- contact dermatoconjunctivitis
- giant papillary conjunctivitis
Seasonal and perennial conjunctivitis
Seasonal and perennial allergic conjunctivitis are usually caused by:
- pollen from grass, trees or flowers
- dust mites
- flakes of dead animal skin
These types of conjunctivitis are more common in people who also have other allergies, such as asthma, and often occur with allergic rhinitis.
Contact dermatoconjunctivitis is usually caused by eye drops, but it can also be caused by make-up or chemicals.
Giant papillary conjunctivitis
Giant papillary conjunctivitis is caused by:
- contact lenses
- stitches used in eye surgery
- a prostheses (artificial) part of the eye that's fitted during eye surgery
Giant papillary conjunctivitis is estimated to affect around 1-5% of people who use soft contact lenses and 1% of people who use hard contact lenses.
Irritant conjunctivitis can have a wide range of potential causes. Some common causes include:
- chlorinated water used in swimming pools
- a stray eyelash rubbing against the conjunctiva
- smoke or fumes