Your eyes may offer a “mirror to your soul,” as the poet says, but everyone else knows they are among the most fragile of our crucial organs.

Your eye's first line of defense is its cornea, the dome-shaped tissue that overlays the pupil and the iris. The cornea keeps bacteria, viruses, fungi, parasites and all manner of bad business away from your eye's more sensitive inner workings, but it is not immune to these itself. Any kind of incursion upon your cornea, whether infectious or not, is known as keratitis. Left untreated, the complications can eventually affect your vision.

Keratitis can be caused by a simple, minor injury, such as what occurs when something scratches or penetrates your cornea. If the injury allows fungi or bacteria to penetrate as well, the keratitis is said to be “infectious.” This condition is frequently the result of wearing contaminated contact lenses.

Contaminated water found in either poorly-cleaned swimming pools or in bacteria- and fungi-filled fresh watering holes can likewise causeinfectious keratitis. In most circumstances your corneas can withstand the onslaught of infectious microbes the world throws at them – that's what they're built for, after all. But if the outer layer of one of your corneas – called its “epithelium” – has been broken down by, say, wearing your contact lens too long, it may be open to infection. Keratitis is more common in people who favor “extended wear” lenses for this reason. Use of corticosteroid eyedrops can also lower your corneas' resistance to the disease.

Symptoms of keratitis may include:

• Eye Pain
• Excess tears or other discharge from your eye
• Eye redness
• Blurred vision
• Difficulty opening your eyelid because of pain or irritation
• Sensitivity to light
• Decreased vision
• A feeling that something is in your eye

If you experience any of these, see your doctor immediately. She will perform a general examination of your eye, usually employing a light source such as a penlight or slit lamp, to illuminate and gauge the reaction of your eye's pupil, iris, cornea and lens. Your doctor may also take a sample of your tears for analysis.

If you are diagnosed with non-infectious keratitis – usually the result of a physical scratch from extended lens wear – your doctor may elect to do nothing, or prescribe drops to relieve the condition.  If you are diagnosed with infectious keratitis, however, the treatment will vary with the source of infection, and may take the form of both eyedrops and oral antibiotics. The worst case scenario is acanthamoeba keratitis, which is caused by a tiny parasite that is often resistant to medication.

Sources: The Mayo Clinic