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Conjunctivitis is an infection of your conjunctiva. Your conjunctiva is a thin surface that covers the inside of your eyelids and the whites of your eyes.

If bacteria infect your conjunctiva, it can become inflamed. This is called bacterial conjunctivitis. Your eyes look red and feel irritated, uncomfortable and watery. You can also get thick pus coming from your eyes.^{1 2}

Lots of things, including dust, sand and pollen, can irritate your eyes and make them inflamed. Viruses and allergies can also cause conjunctivitis.¹ But if you have conjunctivitis caused by bacteria, you'll wake up with crusty eyelids.^{2 3} This is caused by the pus coming from your eyes while you sleep. It's usually worse than the "sleep" you normally have in your eyes when you wake up.

Conjunctivitis caused by viruses or bacteria can easily spread from person to person. You can help prevent this by:

• Washing your hands often
• Trying not to touch your eyes.

Conjunctivitis is very common among school children. Viruses and bacteria can spread from child to child when they play.¹ Children are also much more likely to get conjunctivitis if they already have an ear infection. The bugs that cause ear infections and sinusitis can also cause conjunctivitis.

Adults are much more likely to get conjunctivitis if they tend to have very dry eyes.⁴

In about 65 out of 100 people, conjunctivitis clears up on its own after a few days.⁵ But conjunctivitis can occasionally be a sign of more serious eye problems, such as glaucoma or an infection in your eyelid (a stye). And the infection that causes the conjunctivitis can sometimes get worse and spread if it isn't treated.¹

Doctors don't often do tests to find out if your conjunctivitis is caused by bacteria. This is because the conjunctivitis usually clears up before the test results come back. Some doctors treat all people with conjunctivitis with antibiotic eye drops or ointment. Other doctors only treat conjunctivitis with antibiotics if they think it's been caused by bacteria.²

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.