1 · What is keratoconus? Keratoconus is a progressive disease affecting the front window of the eye, the cornea, and results in poor vision that cannot be corrected fully with glasses. Keratoconus usually begins in the late teenage years. However, it can start in the 20s or early 30s. Keratoconus causes the "clear window" at the front of the eye to become thin and bow outwards.