Pink eye in adults is most often caused by a virus, with viral infections responsible for roughly 80% of all acute cases. Bacterial infections, allergies, and environmental irritants account for the rest. Each type has distinct triggers and behaves differently, so knowing what caused yours helps you understand how long it will last, whether it’s contagious, and what to do about it.
Viral Infections: The Most Common Cause
Adenoviruses cause approximately 80% of viral conjunctivitis cases. These are the same family of viruses behind many common colds, which is why pink eye so often shows up alongside a sore throat, runny nose, or upper respiratory infection. You can pick up the virus by touching a contaminated surface and then rubbing your eye, sharing towels or pillowcases, or simply being in close contact with someone who’s infected. Herpes simplex virus accounts for a smaller share, roughly 1.3 to 4.8% of acute cases, but tends to be more serious and can recur.
Viral pink eye typically starts in one eye and spreads to the other within a day or two. The discharge is usually watery rather than thick, and your eyes may feel gritty or burning. Symptoms can appear anywhere from 12 hours to 12 days after exposure, which makes it easy to forget where you picked it up. The infection stays contagious as long as your eyes are tearing and producing discharge, often around one to two weeks.
Bacterial Infections
Bacterial conjunctivitis is less common than viral but produces more dramatic symptoms. The hallmark is a thick, yellow or greenish discharge that can crust your eyelids shut overnight. Several species of bacteria are responsible, including Staphylococcus aureus (a common skin bacterium), Streptococcus pneumoniae, and Haemophilus influenzae. Symptoms usually appear 24 to 72 hours after exposure, a noticeably faster onset than viral cases.
Adults often contract bacterial pink eye by touching their eyes with unwashed hands, using contaminated eye makeup, or sharing personal items like towels. Contact lens wearers face additional risk. Sleeping in lenses, rinsing them with tap water, or reusing old solution creates a warm, moist environment where bacteria thrive. The CDC classifies contact lenses as medical devices and notes that failure to clean and store them properly significantly increases the chance of eye infections.
Allergies and Seasonal Triggers
Allergic conjunctivitis looks similar to the infectious types but has a completely different mechanism. Your immune system mistakenly treats a harmless substance (pollen, dust mites, mold spores, or pet dander) as a threat and launches a defensive response. That response floods the tissue lining your eye with inflammatory chemicals, producing redness, intense itching, and watery eyes. Unlike infectious pink eye, the allergic version almost always affects both eyes at once and is not contagious.
Seasonal allergic conjunctivitis flares during spring and fall when pollen counts are high. Perennial allergic conjunctivitis, triggered by indoor allergens like dust mites and pet dander, can linger year-round. Some people also react to chemicals and fragrances in everyday products: soaps, detergents, deodorants, moisturizers, and perfumes can all trigger symptoms in sensitive individuals.
Chemical and Environmental Irritants
You don’t need an infection or allergy to get pink eye. Direct exposure to irritating substances can inflame the conjunctiva on its own. Chlorine is a common culprit, but the real irritant in pools is often chloramines, compounds that form when chlorine reacts with sweat, dirt, body oils, and urine in the water. Chloramines can also become airborne around indoor pools, irritating your eyes even above the waterline.
Cigarette smoke, air pollution, dust, chemical fumes, and aerosol sprays are other frequent triggers. This type of pink eye usually clears once you remove the irritant and flush your eyes with clean water. It’s not contagious and doesn’t require antibiotics.
Contact Lenses as a Risk Factor
Contact lens wear deserves special attention because it amplifies the risk of both bacterial and irritant conjunctivitis. A lens sits directly on your cornea, trapping bacteria against the eye’s surface and reducing the natural flow of tears that normally wash germs away. Swimming or showering in contacts exposes your eyes to waterborne organisms that can adhere to the lens. Even cosmetic or decorative lenses carry the same risks if they aren’t properly cleaned.
The simplest protective steps are washing your hands before handling lenses, replacing solution in the case daily rather than topping it off, and removing lenses before swimming or sleeping (unless they’re specifically designed for overnight wear).
Autoimmune Conditions and Underlying Disease
Recurring or persistent pink eye in adults sometimes points to an underlying autoimmune condition rather than a simple infection. Reactive arthritis (formerly called Reiter’s syndrome), psoriatic arthritis, and systemic lupus erythematosus can all produce conjunctivitis as one of their outward symptoms. Rheumatoid arthritis can cause a related condition called episcleritis, which produces a bright red eye that’s easily confused with conjunctivitis but involves deeper tissue.
If your pink eye keeps coming back without an obvious trigger, or if it accompanies joint pain, skin rashes, or other systemic symptoms, the cause may be inflammatory rather than infectious. A thorough history and eye exam can help distinguish between these possibilities.
How to Tell Which Type You Have
The type of discharge is the most useful clue. Watery, clear discharge alongside cold symptoms strongly suggests a viral cause. Thick, colored discharge that glues your eyelids together overnight points toward bacteria. Intense itching with watery eyes in both eyes, especially during allergy season, is the signature of allergic conjunctivitis. Irritant-related cases tend to produce burning and tearing that started shortly after a specific exposure, like a pool visit or contact with fumes.
Viral pink eye typically lasts one to three weeks and resolves on its own. Bacterial cases often improve within a few days of starting antibiotic eye drops, though mild cases can also clear without treatment. Allergic conjunctivitis persists as long as you’re exposed to the trigger but responds well to antihistamine drops or avoidance strategies. If your vision changes, you develop significant pain, or symptoms haven’t improved after a week, that warrants a closer look from a professional.