Pink eye, known medically as conjunctivitis, is caused by viruses, bacteria, allergens, or chemical irritants that inflame the thin membrane covering the white of your eye. Viruses account for up to 80% of the roughly 6 million acute cases that occur in the United States each year, making them the most common culprit by a wide margin. The rest are split among bacterial infections, allergic reactions, and environmental irritants.
Viral Infections: The Most Common Cause
Adenoviruses are responsible for the vast majority of viral pink eye cases. There are at least 51 distinct types of adenovirus, and different strains cause different levels of severity. Some trigger a mild, short-lived redness that resolves on its own. Others cause epidemic keratoconjunctivitis, a more aggressive form that can spread rapidly through schools, offices, or households. The risk of passing it to family members living in the same home ranges from 10% to 50%.
The virus spreads through contaminated hands, shared towels, tissues, swimming pools, and possibly airborne droplets from sneezing. It can also survive on surfaces like doorknobs and medical instruments long enough to infect the next person who touches them and then rubs their eye. Herpes simplex virus and varicella-zoster virus (the same virus behind chickenpox and shingles) can also cause pink eye, though these cases are less common and tend to be more serious.
Viral pink eye typically clears up in 7 to 14 days without treatment. Antibiotics do nothing against it. Cold compresses and artificial tears can ease discomfort while you wait it out, and antiviral medications are reserved for the more serious herpes-related cases.
Bacterial Infections
Bacterial pink eye looks and feels different from the viral version. It tends to produce thicker discharge, often yellow or green, that crusts over your eyelashes overnight. The infection can affect one or both eyes.
The bacteria behind it vary by age. In children, the most frequent causes are Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis. In adults, Staphylococcus aureus is a leading cause, particularly in chronic cases that linger for weeks. Sexually active adults can also develop conjunctivitis from Chlamydia trachomatis, which usually affects only one eye and produces heavy pus-like discharge.
Mild bacterial pink eye often resolves on its own in 2 to 5 days, though it can take up to two weeks to fully clear. Antibiotic eye drops or ointment can shorten the infection, reduce the chance of spreading it, and are especially important if you have a weakened immune system or heavy discharge.
Allergic Reactions
If your pink eye comes with intense itching but no thick discharge, an allergen is the likely trigger. Pollen, pet dander, dust mites, and mold spores are the most common offenders. Unlike viral and bacterial forms, allergic conjunctivitis is not contagious.
The reaction happens because your immune system overreacts to a harmless substance. Specialized immune cells in the conjunctiva release histamine and other inflammatory chemicals, which cause the blood vessels in your eye to swell and leak fluid. That produces the characteristic redness, watering, and puffiness. The conjunctiva is especially prone to this kind of reaction because it sits exposed to the air and has a rich blood supply, making it one of the first tissues to encounter airborne allergens.
Removing the allergen is the most effective fix. Over-the-counter antihistamine eye drops and oral allergy medications can provide additional relief. Some people experience seasonal flare-ups every spring or fall, while others deal with year-round symptoms tied to indoor allergens like dust or pet hair.
Chemical Irritants and Air Pollution
Your eyes can become red and inflamed without any infection or allergic response. Chlorine in swimming pools, household cleaning products, smoke, and cosmetics that get into the eye can all irritate the conjunctiva directly. This type of irritant conjunctivitis usually resolves once you flush the substance out with clean water or artificial tears.
Air pollution is an underappreciated cause. A systematic review found that six common air pollutants, including nitrogen dioxide, ozone, sulfur dioxide, and particulate matter, all correlate with higher rates of conjunctivitis. Nitrogen dioxide had the strongest effect. It is an acidic gas that disrupts the chemical balance of cells on the eye’s surface, triggering inflammation. Ozone is similarly oxidizing and irritating. People who live near heavy traffic or in cities with poor air quality may experience chronic low-grade eye redness and irritation tied to these exposures.
Pink Eye in Newborns
Newborns can develop conjunctivitis during delivery if the mother carries certain infections. Chlamydia trachomatis is the most common infectious cause, responsible for about 35% of neonatal cases. Neisseria gonorrhoeae, the bacterium behind gonorrhea, accounts for around 10% but carries a higher risk of permanent eye damage if untreated. Common bacteria like Staphylococcus, Streptococcus, and Haemophilus cause roughly half of all neonatal cases.
To prevent these infections, the World Health Organization recommends applying antibiotic ointment or antiseptic solution to both of a newborn’s eyes shortly after birth. Silver nitrate was historically the standard but has fallen out of favor because it causes a temporary chemical irritation of its own. Erythromycin ointment is now more commonly used in many countries.
Autoimmune and Systemic Conditions
Recurring or stubborn pink eye that doesn’t respond to standard treatment can sometimes signal an underlying autoimmune condition. Sjögren’s syndrome, one of the more common systemic autoimmune diseases, primarily affects women and attacks the glands that produce tears and saliva. The resulting severe dry eye can lead to chronic conjunctival inflammation.
Rheumatoid arthritis and lupus can also produce eye surface inflammation as a secondary effect. Rarer conditions like ocular cicatricial pemphigoid cause the immune system to attack the basement membrane of the conjunctiva itself, leading to scarring that can threaten vision over time. Stevens-Johnson syndrome, a severe reaction typically triggered by medications or infections, causes widespread inflammation of mucous membranes including the eyes and can result in permanent corneal damage.
How to Tell the Difference
The type of discharge and the specific symptoms you experience offer the strongest clues about what’s causing your pink eye. Viral conjunctivitis tends to start in one eye and spread to the other within a day or two, producing watery, clear discharge along with light sensitivity. It often accompanies a cold or upper respiratory infection.
Bacterial conjunctivitis produces thicker, colored discharge that mats your eyelashes shut in the morning. It may stay in one eye or spread to both. Allergic conjunctivitis almost always affects both eyes simultaneously and causes pronounced itching, which is less common in infectious forms.
Irritant conjunctivitis is usually the easiest to identify because the redness and tearing start shortly after a specific exposure, like swimming in a chlorinated pool or getting soap in your eye. It resolves once the irritant is gone. Pink eye that keeps coming back, especially without an obvious trigger, is worth investigating for an underlying condition like dry eye disease or an autoimmune disorder.