The World Health Organization, based on the 2002 global population survey, shows that blindness resulting from infectious diseases is decreasing. However, blindness from conditions relating to aging is increasing.

According to the State of the World's Sight, VISION 2020: the Right to Sight 1999-2005 published by the World Health Organization in collaboration with the International Agency for the Prevention of Blindness, the major causes of blindness are:

Cataract

Approximately 17.6 million are blind from cataract, a clouding of the lens inside the eye due primarily to aging. It is the leading cause of treatable blindness in developing countries where cataract surgical services are limited. What is Cataract?

Glaucoma

Glaucoma, with an estimated 4.5 million people blind, is the second leading cause of blindness worldwide. The US National Eye Institute reports that glaucoma, which affects 2.2 million people in the US, is the most common cause of blindness among Hispanics in the United States and among African Americans. Glaucoma runs in families and many people are unaware they have this disease. Most cases of glaucoma can be controlled before sight is lost, and this is why examination and early detection are so important. What is Glaucoma?

Trachoma

The WHO estimates there are 84 million people infected with trachoma of whom 1.3 million are blind. This highly infectious disease is caused by the organism Chlamydia trachomatis and is found primarily in 55 countries in Africa, the Middle East, Central Asia, India and South-East Asia. Rates are being slowly reduced due to improved access to water, good hygiene, and antibiotic treatments.

Xerophthalmia

The blinding complication of vitamin A deficiency, is seen most often in malnourished children. Globally an estimated 750 million people have vitamin A deficiency. An estimated 2.7 million, mostly children, are blind of whom 60% will die. Prevention through nutrition education, promotion of home gardens, and the distribution of vitamin A supplements can prevent xerophthalmia, save sight, and prevent early death.

Age-related Macular Degeneration (AMD)

Due to the growing number of people over 70 years of age, AMD ranks third as a cause of global vision impairment, with an estimated 3.2 million blind from this condition. It is the primary cause of visual impairment in industrialized nations, such as the USA. What is AMD?

Diabetic Retinopathy

The blinding complication of diabetes is diabetic retinopathy accounting for an estimated 1.8 million blind people worldwide. It is the leading cause of blindness among working age adults. Keeping diabetes under control through diet and medical treatment is the best way to prevent ocular complications. In severe cases, treatment includes laser and/or surgery. What is Diabetic Retinopathy?

Other major causes of blindness worldwide include ocular injuries, onchocerciasis (river blindness) caused by a parasitic worm carried by the black fly Simulium damnosum, and Hanson's Disease (leprosy).
WHO estimates that cataract, glaucoma, and macula degeneration — all linked to aging — are responsible for up to 75% of the world's blindness. By the year 2020, the UN estimates there will be a global elderly population of 698 million which could significantly raise levels of vision impairment worldwide — hence the importance attached to the "Global Initiative for the Elimination of Avoidable Blindness — Vision 2020."