Sometimes, the cornea is damaged when a foreign object penetrates the tissue, such as if the eye is poked. At other times, bacteria or fungi from a contaminated contact lens can pass into the cornea. Situations like these can cause painful inflammation and corneal infections called keratitis. These infections can reduce visual clarity, produce corneal discharges, and perhaps erode the cornea. Corneal infections can also lead to corneal scarring, which can impair vision and may require a corneal transplant.
As a general rule, the deeper the corneal infection, the more severe the symptoms and complications. It should be noted that corneal infections, although relatively infrequent, are the most serious complication resulting from wearing contact lenses.
Minor corneal infections are commonly treated with antibacterial eyedrops. If the problem is severe, it may require more intensive antibiotic or antifungal treatment to eliminate the infection, as well as steroid eyedrops to reduce inflammation. Frequent visits to an eye care professional may be necessary for several months to eliminate the problem.
This answer is based on source information from the National Institute of Diabetes and Digestive and Kidney Diseases.