Pink eye, known medically as conjunctivitis, has several distinct causes: viruses, bacteria, allergens, irritants, and sometimes underlying health conditions. The most common cause in adults is a viral infection, and most of these cases clear up on their own without treatment. Knowing which type you’re dealing with matters because it determines whether you’re contagious, whether you need medication, and how long it will last.
Viral Infections
Viruses are behind the majority of pink eye cases in adults. Adenoviruses are the most frequent culprit, the same family of viruses that causes the common cold. This is why pink eye often shows up alongside a sore throat, runny nose, or upper respiratory infection. You can catch it through close personal contact like handshakes, airborne droplets from coughing and sneezing, or by touching a contaminated surface and then rubbing your eyes. Poorly chlorinated swimming pools and lakes are less common routes but still possible.
Viral pink eye typically produces a watery, clear discharge and affects one eye first before spreading to the other. It does not respond to antibiotics. Most cases resolve within one to two weeks on their own. More serious viral forms caused by herpes simplex or varicella-zoster (the chickenpox virus) can require antiviral medication, but these are far less common.
Bacterial Infections
Bacterial pink eye tends to look and feel different from the viral type. The hallmark is a thick, yellow-green discharge that can mat your eyelids shut overnight. It often affects one eye, though it can spread to both.
The specific bacteria involved depend on your age. In children, the most common species are ones that also cause ear infections and sinus infections. In adults, staph bacteria are a frequent cause, particularly in chronic cases that linger for weeks. A more aggressive form, caused by the same bacteria responsible for gonorrhea, produces severe, rapid-onset symptoms with heavy discharge and requires urgent treatment to prevent damage to the cornea.
Contact lens wearers face a higher risk of bacterial pink eye and a more dangerous complication: bacterial infection of the cornea itself. If you wear contacts and develop pink eye symptoms, remove your lenses immediately. Topical antibiotics are typically prescribed, and an eye doctor should evaluate you promptly.
Pink Eye in Newborns
Newborns can develop pink eye from bacteria or viruses passed from the mother during delivery. This happens even when the mother has no active symptoms at the time of birth. Chlamydia and gonorrhea are the two most concerning causes because they can lead to serious eye damage if untreated. Other bacteria that naturally live in the birth canal, as well as viruses like herpes, can also cause neonatal pink eye. This is one reason hospitals routinely apply antibiotic ointment to newborns’ eyes shortly after birth.
Allergens
Allergic conjunctivitis is extremely common and not contagious at all. It happens when your immune system overreacts to a harmless substance. Seasonal triggers include tree pollen in spring, grass pollen in summer, and weed pollen in fall. Year-round triggers include dust mites, mold spores, and pet dander.
The key difference from infectious pink eye is intense itching. Both eyes are almost always affected, and you’ll often have other allergy symptoms like sneezing or a runny nose. The discharge tends to be watery rather than thick. Chemicals and fragrances in soaps, detergents, deodorants, and moisturizers can also trigger it, blurring the line between an allergic reaction and simple irritation.
Irritants and Foreign Substances
Your eyes can become red and inflamed without any infection or allergy. Chlorine in pools, cigarette smoke, fumes, dust, and even shampoo that gets into your eyes can all cause irritant conjunctivitis. This type usually resolves once the irritant is removed and the eyes are rinsed with clean water. If redness and discomfort persist after flushing, something else may be going on.
Contact Lenses
Contact lenses deserve their own mention because they contribute to pink eye in multiple ways. Beyond increasing the risk of bacterial infection, lenses can cause a condition called giant papillary conjunctivitis, where the underside of the upper eyelid develops bumps and becomes chronically inflamed. This is triggered by the physical friction of the lens against the eyelid, protein deposits that build up on lenses over time, or an allergic reaction to the lens material or cleaning solutions.
Soft lenses carry the greatest risk. People who already have asthma or hay fever are more susceptible. Switching to preservative-free lens solutions, replacing lenses more frequently, and never sleeping in your contacts all reduce the chance of this happening.
Autoimmune and Systemic Conditions
Sometimes pink eye is a symptom of something happening elsewhere in the body. Rheumatoid arthritis and lupus can both cause chronic dry eye that presents as conjunctivitis. Sarcoidosis, an inflammatory disease that affects multiple organs, can cause a specific type of follicular conjunctivitis. Thyroid disorders, including both overactive and underactive thyroid, are linked to a form of inflammation along the upper edge of the cornea. If pink eye keeps coming back or won’t resolve with standard treatment, one of these underlying conditions may be the reason.
How Long You’re Contagious
Viral and bacterial pink eye are both contagious. The general rule is that you remain contagious as long as your eyes are tearing and producing discharge, particularly if eyelids are matted. Symptoms typically improve within a few days to two weeks depending on the cause. Allergic and irritant-based pink eye are never contagious.
To avoid spreading it, wash your hands frequently, don’t share towels or pillowcases, avoid touching your eyes, and replace any eye makeup you used while symptomatic.
Symptoms That Need Prompt Attention
Most pink eye is uncomfortable but harmless. However, certain symptoms signal something more serious. Eye pain (not just mild irritation), sensitivity to light, blurred vision that doesn’t clear when you blink, and intense redness all warrant a visit to a healthcare provider. These can indicate a deeper infection, corneal involvement, or a condition that mimics pink eye but requires different treatment entirely.