Conjunctivitis - Causes of conjunctivitis

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.

Infective conjunctivitis

Eye infections are most commonly caused by:

bacteria, for example the strains of bacteria that more commonly cause lung and ear infections
a virus, most commonly an adenovirus that may also cause a sore throat and high temperature (fever)
sexually transmitted infections such as chlamydia or gonorrhoea

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 are more likely to develop infective conjunctivitis if you have been in close contact with someone who is already infected with it.

It is therefore very important to wash your hands thoroughly after coming into contact with anyone who has infective conjunctivitis. Don't share pillows or towels with anyone with the infection.

Risk groups

You may be more at risk of infective conjunctivitis if:

You are old or young: the condition is 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 have 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 prone to infections).
You are taking corticosteroids (steroids), as this medication 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 have been in a crowded place, such as a busy train.

Allergic conjunctivitis

Allergic conjunctivitis is caused when your eyes come into contact with an allergen. An allergen is 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 among people who also have other allergies, such as asthma, and often occur with allergic rhinitis (an allergy that affects the nose, causing sneezing and a blocked or runny nose).

Contact dermatoconjunctivitis

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 (sutures) used in eye surgery
a prostheses (artificial) part of the eye that is 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

Irritant conjunctivitis can have a wide range of potential causes. Some common causes include:

chlorinated water used in swimming baths
shampoo
a stray eyelash rubbing against the conjunctiva
smoke or fumes

Bacteria: Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease and others are good for you.
Conjunctiva: The conjunctiva is the transparent membrane (thin layer of cells) that covers the white part of the eyeball and the inner surfaces of the eyelids.
Immune system: The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Inflammation: Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Respiratory tract: The respiratory tract is a general term that is used to describe all the parts of the body involved in helping a person to breathe.