Corneal ulcers are most commonly caused by an infection with bacteria, viruses, fungi or amoebae. Other causes are abrasions or foreign bodies, inadequate eyelid closure, severely dry eyes, severe allergic eye disease, and various inflammatory disorders.

Contact lens wear, especially soft contact lenses worn overnight, may be a precipitating factor. Herpes simplex keratitis is a serious viral infection. It may have recurrences that are triggered by stress, exposure to sunlight, or any condition which impairs the immune system.

Fungal keratitis can occur after a corneal injury involving plant material, or in immunosuppressed people. Acanthamoeba keratitis occurs in contact lens users, especially those who attempt to make their own homemade cleaning solutions.

Risk factors are dry eyes, severe allergies, history of inflammatory disorders, contact lens wear, immunosuppression, trauma and generalized infection.