People get pink eye when the thin, clear membrane covering the white of the eye becomes inflamed. The three most common triggers are viruses, bacteria, and allergens, though chemical irritants can cause it too. With roughly 6 million cases occurring in the U.S. each year, it’s one of the most frequent reasons people visit a doctor for an eye complaint.
Viruses Are the Most Common Cause
The majority of infectious pink eye cases are caused by adenoviruses, the same family of viruses responsible for many common colds. The virus reaches the eye through direct contact, entering the conjunctival membrane and hijacking epithelial cells to replicate. As those cells break apart, the immune system kicks into gear, producing the redness, swelling, and watery discharge you see in the mirror.
Viral pink eye tends to feel gritty or sandy, as though something is stuck in your eye. The discharge is usually watery rather than thick, and many people develop sensitivity to light. It often starts in one eye and spreads to the other within a day or two. One particularly aggressive form, epidemic keratoconjunctivitis, spreads so easily it has been traced to contaminated equipment in eye doctors’ offices.
Antibiotics do nothing for viral pink eye. The infection typically resolves on its own within one to three weeks, with cool compresses and artificial tears helping manage discomfort in the meantime.
Bacterial Pink Eye Looks Different
Bacterial conjunctivitis produces a thick, yellow or green discharge that can be dramatic. It’s the type most likely to glue your eyelids shut overnight, leaving a crusty layer on the lashes by morning. Despite looking worse than the viral version, bacterial pink eye usually causes less pain. In children, the bacteria most often responsible are common respiratory germs. In adults, staph bacteria are a frequent culprit, especially in cases that linger for weeks.
Mild bacterial pink eye often clears up in 2 to 5 days without treatment, though it can take up to two weeks to fully resolve. Antibiotic eye drops or ointment can shorten that timeline and reduce the risk of spreading it, which is why doctors typically prescribe them when there’s significant pus-like discharge, a weakened immune system, or suspicion of a more aggressive bacterium.
A less common but important bacterial cause is chlamydia, which reaches the eye through genital-to-eye contact. This type usually affects one eye and produces heavy discharge. It requires specific antibiotic treatment rather than standard eye drops.
How Pink Eye Spreads So Easily
Both viral and bacterial pink eye are highly contagious, and the transmission routes overlap significantly:
- Hand-to-eye contact: The most common route. You touch a contaminated surface, then rub your eye.
- Contaminated objects: Shared towels, pillowcases, makeup, and even eye drop bottles can carry the infection.
- Respiratory droplets: Sneezing and coughing can spread the same viruses that cause pink eye, since adenoviruses infect both the respiratory tract and the eyes.
- Eye discharge and tears: Direct contact with infected tears or the crusty discharge around the eye transfers the pathogen efficiently.
Pink eye generally stays contagious as long as the eye is tearing and producing discharge. This is why schools and daycares often ask children to stay home until symptoms improve. Frequent handwashing and avoiding touching your face are the most effective ways to stop the spread.
Allergies Cause a Different Kind of Pink Eye
Allergic conjunctivitis isn’t an infection at all. It happens when your immune system overreacts to pollen, pet dander, dust mites, or mold. The hallmark symptom is itching, sometimes intense, along with clear, watery discharge and mild redness. Both eyes are almost always affected at the same time, which helps distinguish it from infectious types.
Because it’s driven by allergen exposure rather than a pathogen, allergic pink eye isn’t contagious. It lasts as long as you’re around whatever is triggering it, which means seasonal cases can persist for weeks during high pollen counts. Over-the-counter antihistamine eye drops and avoiding the allergen are the main strategies for relief.
Irritants and Chemicals
Sometimes pink eye has nothing to do with infection or allergies. Anything that gets into your eye can inflame the conjunctiva: pool chlorine, soap, cosmetics, smoke, dirt, or even your contact lenses. Some medications can trigger it as a side effect. If a chemical irritant splashes into your eye, flushing it with warm water for five minutes is the immediate step. This type of pink eye usually resolves once the irritant is removed, though the eye may stay red and uncomfortable for a short time afterward.
How to Tell Which Type You Have
The type of discharge is the most useful clue. Thick, yellow-green pus that crusts on the lashes points toward bacteria. Watery discharge with a gritty, painful feeling and light sensitivity suggests a virus. Clear, watery discharge with intense itching in both eyes is the classic allergic pattern.
That said, these categories blur in practice. Viral pink eye can produce some mucus. Allergic pink eye doesn’t always itch severely. And bacterial cases don’t always produce dramatic discharge. If your symptoms are mild and improving, you can often manage at home with cool compresses and artificial tears. Significant pain, blurred vision that doesn’t clear when you blink, or symptoms that worsen after a few days warrant a professional evaluation, since those can signal something beyond routine pink eye.