context: Keratitis is painful inflammation of your cornea -- the clear covering over the pupil and colored part of your eye. Doctors sometimes use the term "corneal ulcer." It's usually caused by an injury or infection. See your doctor right away if you have a problem with your eyes. Keratitis can become serious quickly. The first sign of keratitis is usually eye pain and redness. Your eye may burn or feel irritated, like you have something in it. You may not even be able to open it. Light may hurt your eyes, and you also may have: Blurry vision Lots of tears or a discharge Swelling Several things can lead to keratitis: Injury: Most of the time, keratitis comes from damage to your cornea. You might have been poked in the eye or scratched your eye with your fingernail. It also can happen if you wear your contact lenses too long or wear lenses that don't fit right. The injury may cause the inflammation, but it can also let in bacteria or a fungus that leads to an infection. Viral infection: This is usually herpes simplex, the chicken pox virus, or the common cold. If you're sick, be careful about touching your eyes and keep your hands clean. Bacterial infection: This happens less often, but it can be a problem for people who wear contact lenses. Bacteria can grow on your contacts or contact case if you don't clean and store them properly. Extended-wear lenses, the kind you sleep in for days or even weeks at a time, are most likely to lead to this. Infection can also come from contaminated eye drops or contact solution. Or it can happen after eye surgery. Parasite: Acanthamoeba is a microorganism that lives just about everywhere -- in the air, soil, and bodies of water. It's even found in tap water. It's harmless most of the time. But it can cause an eye infection, especially if you wear contacts. It's very rare, but also very hard to treat. Fungus: It's also very rare to get a fungal infection in your eye. It usually comes from being scratched in the eye by a branch or from contaminated contacts. Eye surgery also can cause this. Other causes: A lack of vitamin A, some illnesses that involve a problem with your immune system, and diseases that cause very dry eyes can lead to keratitis. Your doctor will look for damage to your cornea. She also may swab under your eyelid to get a sample to test for infection. Don't wear your contacts until you find out what's wrong. If your keratitis is caused by an injury, it usually clears up on its own as your eye heals. You may get an antibiotic ointment to help with symptoms and prevent infection. Infections are treated with prescription eye drops and sometimes antibiotics or antiviral medicine. Keratitis is usually easy to treat and clears up quickly. But if an infection goes deeper than the surface of your cornea, it can leave scars that damage your vision or even cause blindness. If you're sick, wash your hands often and try not to touch your eyes. And if you wear contacts, handle your lenses correctly: Don't sleep in lenses you're supposed to take out every day. Don't swim or shower in your contacts. Wash your hands before touching your contacts or your eyes. Always use fresh solution to clean and store your lenses. Rinse your lens case with contact solution and dry it with a clean tissue. Never store contacts in tap water. Replace your lenses and storage case regularly.