Also known as “pink eye,” conjunctivitis is an inflammation of the eye's outer membrane – called the conjunctiva. It is highly contagious and causes redness, swelling, itching and watering in one or both eyes.
The most common type of conjunctivitis is viral and is usually associated with upper respiratory infections. It should be considered contagious. Viral conjunctivitis often starts in one eye and goes to the other.
If yellowish or greenish discharge accompanies the redness and watering, then bacteria may be the cause of the infection, and antibiotic eye drops are often prescribed. Antibiotics will not cure a viral conjunctivitis which may take from three to 14 days to resolve.
In contrast, allergic conjunctivitis will show itching as its most common symptom. It almost always involves both eyes with a whitish mucus discharge and mild eyelid swelling. Allergic conjunctivitis can be treated with cold compresses and antihistamine eye drops for symptomatic relief, but often recurs due to the chronic nature of the allergic condition.
When a red eye occurs in one eye only, an early bacterial or viral conjunctivitis can be present. However, conditions such as corneal abrasions from trauma or from herpetic corneal disease should be ruled out.