Trachoma is an infection of the eyes with Chlamydia trachomatis. It is the world's leading cause of preventable blindness. Trachoma begins in childhood and progresses over the years as repeated infections cause irritation and scarring on the inside of the eyelid.

Eventually the eyelashes turn in (entropion), rubbing on the cornea at the front of the eye. The scarring on the cornea leads to severe vision loss and blindness, usually by the time the person is 40 to 50 years old.