Trachoma is the world’s leading infectious cause of blindness. It’s easy to prevent, but the World Health Organization estimates 1.5 million people are visually impaired because of the disease.

Trachoma is caused by a bacterial infection, and can be treated with antibiotics. But if it’s not treated, repeated infections cause scar tissue to build up inside the eyelids. This pulls the eyelashes inwards so they scrape against the eye, causing incredible pain and, eventually, sight loss.

Cataracts are caused by a build-up of protein in the lens of the eye, which can lead to blurred vision, sensitivity to light and eventual blindness.

More than 17 million people worldwide are blind because of cataracts. They often affect older people, but in poorer countries, they cause sight loss in thousands of children. Thankfully, a straightforward operation can treat the condition.

Refractive errors are the world’s leading cause of visual impairment. These include long-sightedness, short-sightedness, and astigmatism.

Imperfections in the shape of the eye can cause blurred vision, but glasses can correct them if needed. However, this condition can be a problem in low-income countries, where people may not be able to afford the eye tests or glasses they need to improve their sight.

This parasitic infection causes serious skin irritation and can lead to permanent blindness. However, it can be treated with antibiotics, which stops the infection from spreading and prevents sight loss.

River blindness spreads through bites from infected black flies that live near fast-flowing rivers. This often forces people to move away from fertile river valleys, leaving them unable to grow crops. About 1.1 million people around the world are blind because of this disease.

Glaucoma is caused by a build-up of pressure inside the eye. This damages the optic nerve that connects the eye to the brain.

About 3.6 million people around the world are blind because of glaucoma, making it the second-highest cause of blindness worldwide. The condition can be difficult to diagnose because symptoms develop slowly over many years, and don’t appear straight away.

This eye disease is caused by high blood sugar and high blood pressure. It can damage the blood vessels in the eye, eventually leading to blindness.

People who have diabetes are also at higher risk of developing other eye problems, including cataracts and glaucoma. Keeping blood sugar levels, blood pressure and cholesterol levels under control can prevent diabetes-related vision problems.