Wearing contacts for long period blocks oxygen from reaching your eyes. Also, bacteria on the lens — transferred from your finger when inserting or from non-sterile cleaning solutions — can get trapped under your lens. These factors raise the risk of infection, which can lead to a corneal ulcer.
A contact lens wearer is about 10 times more likely to get a corneal ulcer than someone who doesn’t wear contacts. Someone who sleeps in contact lenses is about 100 times more likely to get an ulcer than someone who doesn’t wear contacts.