Trachoma is a disease of the eye and the leading infectious cause of blindness worldwide. It is caused by an obligate intracellular bacterium called Chlamydia trachomatis. Infection is transmitted from person to person by direct or indirect transfer of ocular and nasal discharges of infected people; indirect transfer includes carriage on the body of species of flies. Preschool-age children harbour the principal reservoir of infection. Models suggest that an individual requires more than 150 lifetime infections to develop the blinding complications of trachoma.