Eye infections are caused by bacteria, viruses, fungi, or parasites that breach the eye’s natural defenses. The most common type, infectious conjunctivitis, accounts for roughly 35% of all eye-related doctor visits, with an annual incidence of about 13 to 14 cases per 1,000 people. The specific cause depends on how the pathogen gets in, which part of the eye it targets, and what conditions give it an opening.
Bacterial Causes
Bacteria are responsible for a large share of eye infections, particularly conjunctivitis (pink eye) and keratitis (infection of the cornea). Staphylococcus species are among the most frequent culprits in everyday bacterial conjunctivitis. You can usually recognize a bacterial infection by its discharge: thick, white-yellow pus that reforms quickly after you wipe it away, often gluing eyelashes together overnight.
A more aggressive bacterium, Pseudomonas aeruginosa, is the most common cause of contact lens-related keratitis and tends to produce worse outcomes than other bacterial infections. It thrives in moist environments, which is why contact lens cases, inadequately chlorinated pools, and hot tubs can all serve as reservoirs. In more serious scenarios, bacteria like coagulase-negative Staphylococcus and related species can cause endophthalmitis, a deep infection inside the eye, typically after eye surgery. The overall rate of this complication after cataract surgery is low, around 0.06% in recent years, and continues to decline.
Viral Causes
Adenovirus is the single most common cause of viral pink eye. It spreads easily through direct or indirect contact with fluid from an infected person’s eye, and outbreaks are frequently traced to swimming pools. Viral conjunctivitis produces a watery, clear discharge rather than the thick pus of bacterial infections, and it often starts in one eye before spreading to the other.
Herpes simplex virus type 1 (HSV-1) causes a more serious form of eye infection. The virus attaches to receptors on the corneal surface using proteins on its outer envelope, then breaks through the protective outer layer to infect deeper tissue. This can lead to herpes stromal keratitis, a recurring condition that damages the cornea’s structural layer and can threaten vision over time. Varicella-zoster virus, the same virus behind chickenpox and shingles, can also infect the eye when it reactivates along nerves that supply the face.
Fungal Causes
Fungal eye infections are less common than bacterial or viral ones but tend to be harder to treat. The most frequent fungal cause of corneal infection is Fusarium, followed by Aspergillus and Curvularia species. These filamentous fungi account for up to one-third of all keratitis cases that follow eye trauma, which is why injuries involving plant material or soil carry a heightened risk. Agricultural workers and people in tropical climates are disproportionately affected.
Candida, a yeast that normally lives on the skin and mucous membranes, is the most common fungal cause of endophthalmitis. Unlike Fusarium infections, which typically enter through a wound, Candida infections often originate from the bloodstream and reach the eye internally. This route is more common in people with compromised immune systems or those receiving prolonged intravenous treatment in hospital settings.
Parasitic Causes
Acanthamoeba, a microscopic organism found in water, soil, and dust, causes one of the most painful and difficult-to-treat corneal infections. It exists in two forms: an active feeding stage and a hardy dormant cyst that resists disinfection. Acanthamoeba has been found in swimming pools, hot tubs, drinking water systems, and even air conditioning units.
Contact lens wearers face the highest risk. Swimming, showering, or using a hot tub while wearing contacts creates an ideal scenario for the organism to get trapped between the lens and the cornea. Poor contact lens hygiene compounds the risk. That said, some people who follow proper lens care still develop the infection, so water exposure alone can be enough.
Contact Lenses as a Risk Factor
Contact lenses deserve their own discussion because they are the single biggest modifiable risk factor for corneal infections. The two most common behaviors behind contact lens-related keratitis are overnight wear, responsible for 43% of cases, and poor hygiene, responsible for 33%. Sleeping in lenses reduces the oxygen reaching your cornea and creates a warm, moist pocket where bacteria, especially Pseudomonas, can multiply rapidly.
The risks aren’t limited to bacteria. Contact lens cases can harbor Acanthamoeba cysts and fungal spores. Rinsing lenses or cases with tap water instead of sterile solution introduces organisms that sterile packaging was specifically designed to keep out. Even “daily wear” lenses carry some risk if you handle them with unwashed hands or expose them to water.
Water Exposure and Swimming
Water is a common vehicle for eye pathogens. Swimming pools are a well-documented source of adenoviral outbreaks, and pools with inadequate chlorination can harbor Pseudomonas and Acanthamoeba. Chlorinated pools carry their own irritant risks: chlorine reacts with organic matter introduced by swimmers (sweat, urine, skin cells, cosmetics) to produce chloramines. Trichloramine is the primary chemical responsible for that familiar stinging sensation in your eyes after swimming, and it damages the protective tear film that normally keeps pathogens out.
Hot tubs are similarly risky. Their warm temperatures promote bacterial growth, and the same organisms found in pools, including Pseudomonas and Acanthamoeba, thrive in them. Fresh water, brackish water, and seawater all contain Acanthamoeba naturally. Any water contact with open eyes increases exposure, but the combination of water and contact lenses multiplies the danger significantly.
How Your Eye Normally Fights Off Infection
Your eyes have several built-in defenses. The tear film contains enzymes that break down bacterial cell walls, along with antibodies that neutralize pathogens on contact. The corneal epithelium, a tightly packed layer of surface cells, acts as a physical barrier. Blinking sweeps debris and microbes toward the inner corner of the eye for drainage.
Infection happens when something disrupts these barriers. A scratch from a contact lens, a piece of debris, or prior surgery gives organisms a foothold. Once a pathogen penetrates the corneal surface, immune cells in the deeper layers detect the invader using specialized receptors. For fungi, the cornea uses a specific receptor called Dectin-1 to recognize fungal cell wall components. The immune response that follows, including swelling, redness, and discharge, is your body’s attempt to contain and clear the infection, but in some cases it also contributes to tissue damage.
Eye Infections in Newborns
Newborns are uniquely vulnerable because their immune systems are immature and they pass through the birth canal, where they can be exposed to maternal infections. The timing of symptoms helps pinpoint the cause. Bacterial infections, particularly from Neisseria gonorrhoeae, tend to appear within the first 48 hours of life. Chlamydia trachomatis, the most common infectious cause of neonatal conjunctivitis in the United States, typically shows up between 5 and 14 days after birth. Herpes virus infections appear in a similar window, around 6 to 14 days.
Chlamydia causes between 2% and 40% of neonatal conjunctivitis cases in the U.S., while gonococcal infections have dropped to less than 1% thanks to routine preventive eye drops given at birth. Those prophylactic drops are effective against gonorrhea but do not prevent chlamydial infections, which require oral treatment. Parents of infants born to mothers with known chlamydia exposure are typically advised to watch for signs of eye or respiratory infection in the weeks following birth.
Post-Surgical Infections
Any eye surgery carries a small risk of introducing bacteria into the interior of the eye, causing endophthalmitis. After cataract surgery, the most common procedure, the infection rate has fallen from about 0.097% before the year 2000 to roughly 0.063% in the most recent data. Preventive antibiotic treatment given during surgery cuts the rate approximately in half, to about 0.045%. The bacteria responsible are usually organisms that normally live on the skin around the eye, especially coagulase-negative Staphylococcus. Fungal endophthalmitis after surgery is rarer and primarily involves Candida species.
Trauma-related infections follow a different pattern. When an object penetrates the eye, it can introduce soil-dwelling fungi like Fusarium or environmental bacteria directly into the wound. Agricultural injuries are particularly high-risk because plant matter carries a heavy fungal load.