Pink eye, known medically as conjunctivitis, is caused by viruses, bacteria, allergens, or chemical irritants. Viruses are the most common culprit in adults, while bacteria cause a larger share of cases in children. Understanding which type you’re dealing with matters because the symptoms, contagion risk, and treatment differ for each.
Viral Infections
Adenoviruses are the leading cause of viral pink eye. These viruses spread through close personal contact like handshakes, through respiratory droplets from coughs and sneezes, and by touching contaminated surfaces and then touching your face. They can also spread through inadequately chlorinated swimming pools and lakes.
Viral pink eye typically produces a clear, thin, watery discharge. It often starts in one eye and spreads to the other within a day or two. The infection tends to accompany or follow a cold or upper respiratory infection. It remains contagious as long as the eyes are tearing and producing discharge, and viruses can survive for extended periods on doorknobs, towels, and other household surfaces. If you touch your eye and then touch a shared object, you can easily pass it along.
Bacterial Infections
Several types of bacteria cause pink eye, and the specific ones vary by age group. In children, the most common bacteria are Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis. In adults, Staphylococcus aureus is a frequent cause, particularly in chronic cases that linger for weeks.
The hallmark of bacterial pink eye is a thick, yellow or greenish discharge that crusts around the eyes and can glue your eyelids shut overnight. This discharge is noticeably thicker than what viral pink eye produces. Like viral forms, bacterial pink eye spreads through direct contact and contaminated surfaces, and it stays contagious as long as the eyes are tearing and matted.
A more aggressive form, called hyperacute bacterial conjunctivitis, is caused by the bacteria responsible for gonorrhea. This type produces heavy pus, progresses rapidly, and can damage the eye if untreated. Sexually transmitted bacteria, including chlamydia, can also cause pink eye in adults. People with chlamydial conjunctivitis often have an associated genital infection they may not know about.
Allergens and Seasonal Triggers
Allergic pink eye is not contagious. It happens when your immune system overreacts to something in the environment. Seasonal allergic conjunctivitis is triggered by pollen, grass, and other airborne allergens that peak in spring and fall. A year-round form, called perennial allergic conjunctivitis, is set off by indoor allergens like animal dander, dust, and mold spores.
The symptoms are distinct from infectious pink eye: intense itching in both eyes, along with redness, swelling, and excessive tearing. Both eyes are almost always affected at the same time, which helps distinguish it from viral or bacterial types that often start in just one eye.
Chemical Irritants
Exposure to certain chemicals can inflame the conjunctiva without any infection involved. Swimming pools are a common source. Chlorine itself is relatively mild, but when it combines with sweat, urine, dirt, skin cells, and makeup that wash off swimmers’ bodies, it forms compounds called chloramines. These irritants cause the red, stinging eyes many people experience after swimming. Smoke, fumes, and cosmetics applied near the eyes can trigger similar reactions.
Contact Lenses
Contact lens wear is a significant and often overlooked cause of pink eye. About 5% of soft contact lens wearers develop a specific type called giant papillary conjunctivitis, where bumps form on the inner surface of the upper eyelid. This can result from allergic reactions to the lenses themselves or to cleaning solutions, friction from lenses rubbing against the eyelid, or protein and pollen deposits building up on the lens surface. Non-disposable lenses carry the highest risk.
You can lower this risk by switching to daily disposable lenses, avoiding lens solutions with preservatives, never sleeping in your contacts, and physically rubbing and rinsing your lenses during cleaning rather than just soaking them. Always wash your hands before handling lenses.
Pink Eye in Newborns
Newborns can develop pink eye from bacteria they encounter during delivery. Gonorrhea and chlamydia infections in the mother’s birth canal are the most serious causes. Gonococcal eye infection in a newborn is a medical emergency because it can perforate the eye and cause blindness. This is why hospitals routinely apply antibiotic ointment to newborns’ eyes shortly after birth and why screening pregnant women for these infections is standard practice. Chlamydial conjunctivitis in newborns tends to develop a bit later, usually within the first few weeks of life.
How to Tell the Types Apart
The type of discharge is the most useful clue. Clear, watery discharge that accompanies cold symptoms points to a virus. Thick, yellow-green discharge that crusts overnight suggests bacteria. Intense itching in both eyes with watery tearing, especially during allergy season, signals an allergic cause. Irritation that started after swimming, exposure to fumes, or using a new cosmetic product points to a chemical irritant.
Pink eye from viruses and bacteria spreads through direct contact with infected eye secretions and through contaminated surfaces like towels, pillowcases, and doorknobs. The infection remains contagious for as long as the eyes are watery and producing discharge. Allergic and chemical pink eye cannot spread to other people.