Eye infections fall into four broad categories based on what causes them: bacterial, viral, fungal, and parasitic. But the more practical way to understand them is by where in the eye they strike, because location determines how serious an infection is and how it gets treated. Some are minor nuisances that clear up on their own. Others are emergencies that can cost you your vision within hours.
Conjunctivitis (Pink Eye)
Conjunctivitis is the most common eye infection. It affects the conjunctiva, the thin clear tissue that covers the white of your eye and lines the inside of your eyelids. The telltale signs are redness, swelling, and discharge, but the type of discharge tells you a lot about the cause.
Bacterial conjunctivitis produces thick, yellowish or greenish pus that mats your eyelids together overnight. You may wake up unable to open one or both eyes without washing the crust away first. It can also cause blurred vision and eyelid swelling. This type typically needs antibiotic eye drops to resolve.
Viral conjunctivitis is more watery than goopy. It often shows up alongside a cold, sore throat, or flu, and adenoviruses are the most frequent culprit. Most cases resolve on their own without any treatment. Herpes simplex virus can also cause conjunctivitis, though this form is less common and more serious because it can spread deeper into the eye.
Allergic conjunctivitis looks similar but isn’t an infection at all. Intense itching is the giveaway. If your eyes itch more than they hurt, allergies are the more likely explanation.
Keratitis (Corneal Infections)
Keratitis is an infection of the cornea, the clear dome that covers the colored part of your eye. It’s more serious than conjunctivitis because the cornea is essential for focusing light. Untreated keratitis can scar the cornea and permanently reduce your vision.
The two bacteria most commonly responsible are Pseudomonas aeruginosa, found in soil and water, and Staphylococcus aureus, which lives naturally on human skin and mucous membranes. Viral keratitis is most often caused by herpes simplex, the same virus behind cold sores. Symptoms include pain, redness, sensitivity to light, and the feeling that something is stuck in your eye.
Contact lens wearers face the highest risk for keratitis. Specific habits that increase that risk include sleeping in lenses, rinsing or storing lenses in tap water, topping off old solution instead of replacing it, not cleaning lens cases, and sharing decorative or colored lenses. A single night of sleeping in contacts raises your odds significantly.
Acanthamoeba Keratitis
This rare but severe form of keratitis deserves special attention. It’s caused by Acanthamoeba, a microscopic parasite found in tap water, pools, hot tubs, and soil. It almost exclusively affects contact lens wearers. Symptoms include severe eye pain, the sensation of a foreign body in the eye, and a whitish halo around the edge of the cornea. Treatment is difficult and prolonged, often taking months.
The EPA specifically warns against using tap water, homemade saline solutions, or saliva to clean contacts. Swimming or showering while wearing lenses is another common route of exposure. People who buy contacts without a prescription, such as at flea markets or novelty shops, are also at elevated risk because they often skip proper lens care entirely.
Eyelid Infections
Three conditions affect the eyelids, and they’re easy to confuse with each other.
A stye is a small, painful lump that forms at the base of an eyelash or just under the eyelid. Most styes are caused by a bacterial infection in either a hair follicle (external stye) or an oil-producing gland (internal stye). They tend to appear right at the eyelid’s edge and are noticeably tender to the touch. Most resolve within a week or two with warm compresses.
A chalazion looks similar but develops farther back on the eyelid and is usually painless. It forms when an oil gland gets clogged, not infected. Chalazia can linger for weeks and sometimes need minor treatment to drain if they don’t shrink on their own.
Blepharitis is a broader condition involving chronic redness and swelling along the base of the eyelashes. It’s not a single acute infection but an ongoing irritation that makes you more prone to developing styes and chalazia over time. Good eyelid hygiene, like daily warm compresses and gentle lid scrubs, is the main way to manage it.
Uveitis (Middle Layer Infections)
Uveitis is inflammation of the uvea, the middle layer of the eye that includes the iris and the blood-vessel-rich tissue lining the back of the eye. It can be triggered by infection, injury, or autoimmune disease. In roughly half of all cases, no specific cause is ever identified.
There are four types based on location. Anterior uveitis, the most common form, affects the front of the eye between the cornea and the iris. Intermediate uveitis involves the gel-filled center of the eye and the retina behind the lens. Posterior uveitis targets the retina or the tissue layer beneath it at the back of the eye. Panuveitis means all layers are inflamed from front to back.
When a cause can be pinpointed, it’s often an autoimmune or inflammatory condition affecting other parts of the body: sarcoidosis, lupus, Crohn’s disease, or ankylosing spondylitis (a type of spinal arthritis for which uveitis is one of the most common complications). Very rarely, lymphoma or another cancer affecting the eye is responsible. Symptoms typically include eye pain, redness, light sensitivity, and blurred vision.
Endophthalmitis (Inside the Eye)
Endophthalmitis is an infection of the fluid or tissue inside the eyeball. It is rare, but it’s the most dangerous eye infection because it can destroy vision rapidly. Acute endophthalmitis is a medical emergency.
It most commonly develops after eye surgery, such as cataract removal or corneal transplant, or after a penetrating eye injury. Fungal species in the Candida family cause about 66% of endophthalmitis cases following corneal transplant surgery. Symptoms include worsening eye pain after surgery or injury, significant redness, discharge, swollen eyelids, light sensitivity, and sudden vision loss. If you experience increasing pain after any eye procedure rather than gradual improvement, that warrants immediate contact with your eye care provider.
How Eye Infections Are Diagnosed
An eye care provider can often identify the type of infection based on your symptoms, history, and a careful examination with a slit lamp, which is essentially a high-powered microscope with a bright light. For corneal problems, a test called fluorescein staining is common: a strip of blotting paper containing orange dye is touched to the surface of your eye, you blink to spread it, and the provider shines a blue light. Any damage or infection on the cornea glows green, and the size, shape, and location of the staining help pinpoint the cause. In more serious or unclear cases, a sample of discharge or tissue may be cultured to identify the exact organism involved.
Reducing Your Risk
Most eye infections are preventable with basic hygiene, especially if you wear contact lenses. The CDC recommends washing your hands thoroughly with soap and water before handling lenses, then drying them completely with a clean cloth. Use only the contact lens solution your eye care provider recommends. Rub and rinse lenses with that solution each time you remove them.
Lens case care matters just as much as lens care. Clean the case by rubbing and rinsing with solution (not water), store it upside down with the caps off to air dry, and replace it at least every three months. Never mix fresh solution with old solution already in the case. Keep your lenses away from all water: remove them before swimming, showering, or using a hot tub. And always keep a pair of backup glasses with your current prescription, so you can take your lenses out the moment something feels off.