Keratitis is an inflammation of the cornea, the outermost part of the eye that covers the pupil and iris (the colored ring around the pupil). The most common causes of keratitis are infection and injury.
Bacterial, viral, parasitic and fungal infections can cause keratitis. An infectious keratitis can happen after an injury to the cornea. But an injury can inflame the cornea without a secondary infection occurring.
Viral keratitis occurs quite commonly and the types of viruses include:
Adenovirus, which is one of the causes of upper respiratory infections.
Herpes simplex type 1, the same virus that causes cold sores.
Varicella zoster (also a herpes virus), which is associated with chickenpox and shingles.
Bacterial keratitis occurs less often than viral keratitis. Parasitic and fungal keratitis is rarely seen in developed countries.
Infectious keratitis usually begins by affecting the outer layer of the cornea, but it can go deeper into the cornea, increasing the risk of impaired vision.
Non-infectious keratitis is a feature of some autoimmune diseases, such as rheumatoid arthritis and Sjogren's syndrome.
Trauma to the front of the eye, as may occur with poorly fitting contact lenses, surgery on the cornea (including LASIK surgery), or any other injury to the cornea may lead to keratitis.
People who wear contact lenses are at increased risk for infectious keratitis. Lens wear should stop immediately if a person suspects that he or she is developing an eye infection.