Conjunctivitis (Pink Eye): Sorting Out the Three Main Causes

01

Three Different Causes, One Red Eye

Conjunctivitis is inflammation of the conjunctiva, the clear membrane covering the white of the eye and lining the eyelids. Viral conjunctivitis, often accompanying a cold, is the most common cause and the most contagious, spreading easily through contact with eye discharge or contaminated surfaces. Bacterial conjunctivitis is less common but causes a similar red eye. Allergic conjunctivitis, from pollen, pet dander, or other allergens, isn't contagious at all.

Viral and bacterial conjunctivitis usually affect one eye first and often spread to the other within a day or two; allergic conjunctivitis usually affects both eyes at once and comes with itching more than the gritty discomfort of an infection.

02

Telling Them Apart

Viral conjunctivitis typically causes watery discharge, redness, and a gritty feeling, often alongside cold symptoms. Bacterial conjunctivitis tends to produce thicker, yellow or green discharge that can make the eyelids stick together, especially noticeable after sleep. Allergic conjunctivitis causes itching as its main feature, with watery discharge and often a history of other allergy symptoms.

These patterns overlap enough that a doctor's exam is more reliable than guessing, especially since treatment — or the decision to treat at all — differs meaningfully between the three.

03

How It's Diagnosed

Conjunctivitis is usually diagnosed by the appearance of the eye and the pattern of symptoms, without needing lab testing in typical cases. A doctor may look for signs pointing to a specific cause, such as swollen lymph nodes near the ear (common with viral conjunctivitis) or a history of seasonal allergies.

Eye pain (rather than the milder irritation typical of conjunctivitis), significant light sensitivity, blurred vision that doesn't clear with blinking, or a contact lens wearer with these symptoms all point toward something that needs a more thorough eye exam, since a few more serious conditions — including keratitis, uveitis, or angle-closure glaucoma — can look similar early on.

Pink eye in a newborn (under about a month old) is treated as its own category and should be seen the same day rather than watched at home, since an infection acquired around the time of birth can move quickly and threaten vision.

04

Treatment

Viral conjunctivitis usually resolves on its own within one to two weeks; cool compresses and artificial tears ease discomfort while it runs its course, and antibiotics don't help since there's no bacteria to treat. Bacterial conjunctivitis is treated with antibiotic eye drops, though many mild cases also improve without them.

Allergic conjunctivitis responds to antihistamine eye drops and avoiding the triggering allergen when possible; oral antihistamines can help if allergy symptoms extend beyond the eyes. None of the three should be treated with leftover eye drops from a previous, possibly different, eye infection — leftover steroid drops in particular can significantly worsen some eye infections.

05

Preventing Spread

Viral and bacterial conjunctivitis spread easily through hand-to-eye contact and shared items like towels or pillowcases, so frequent handwashing, avoiding touching or rubbing the eyes, and not sharing towels or eye makeup during an active infection meaningfully reduce spread to others.

Contact lenses should be avoided until symptoms fully clear, and disposable lenses worn during an infection should be discarded rather than reused. Most schools and workplaces have their own guidance on when it's safe to return, generally once discharge has resolved or after starting antibiotics if bacterial.