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Causes of Conjunctivitis

Conjunctivitis is most commonly caused by infection. Almost any germ, whether a bacterium or a virus, may be responsible. Some are more dangerous than others. The common cold can cause conjunctivitis, spreading from the mouth and nose to the eyes, but this is usually far less severe than infection of the eyes with chlamydia or herpes. However, conjunctivitis caused by the common cold is very infectious and can spread rapidly between people. Other bacteria that commonly cause conjunctivitis include staphylococci, streptococci and haemophilus bacteria.

The most common cause of allergic conjunctivitis is hypersensitivity to pollen (hay fever). The pollen irritates the eye and can cause the conjunctiva to swell. Occasionally this can lead to extreme swelling and bulging of the eye membrane.

Conjunctivitis can also result from a wide range of other causes, including:

• Chemicals, dusts, liquids, gases and other environmental contaminants, which can cause an allergic reaction. One of the most common causes is the chemicals in swimming pools. 
• Radiation, especially the ultraviolet in sunlight 
• Too much wind blowing on the eyes
• Occasionally, irritation from eye make-up.

Contact lenses can cause conjunctivitis if they get contaminated with bacteria or, rarely, tiny creatures called amoeba. This should not happen if lenses are stored and handled correctly. It is particularly common in people who wear decorative contact lenses, because they are often not properly trained in hygiene and handling of the lenses. Conjunctivitis can sometimes also occur as a result of mechanical irritation, by the rubbing of the lens itself on the eye surface. This seems to happen more with people who use hard lenses.

Sometimes newborn babies have conjunctivitis, caused by bacteria that they came into contact with during birth. This should be brought to the attention of your GP or health visitor immediately. It is important to find out the cause, because it may be a sexually transmitted infection such as gonorrhoea, for which the mother will need treatment.

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