Cataracts are caused by a build-up of protein that clouds the eye’s lens, which can lead to blurred vision and eventual blindness.

More than 12 million people worldwide are blind because of cataracts. They are often thought to only affect older people, but in poorer countries cataracts cause sight loss for thousands babies and infants. Thankfully, the condition can be treated with a straightforward operation.

Nearly half of visual impairment is caused by refractive error, including long-sightedness, short-sightedness and astigmatism.

These irregularities in the shape of the eye can cause blurred vision, but can be corrected with spectacles if needed. However, the condition can be problematic in lower income countries, where people may not be able to afford sight tests or spectacles to help improve their vision.

Trachoma is the world’s leading infectious cause of blindness: there are 44 countries where people are at risk from the disease.

It starts off as a bacterial infection that can be easily treated. But if it’s not, over time it can cause the eyelashes to scrape against the eye, causing pain and, eventually, sight loss. Worldwide, it's thought that 1.9 million people are blind or visually impaired because of the disease.

This parasitic infection causes severe skin irritation and irreversible blindness, but it can be treated with antibiotics to stop it spreading and prevent sight loss.

It’s thought 1.1 million people worldwide are blind because of the disease, which is spread by the bite of infected flies that breed near fast-flowing rivers. This often forces people to move away from fertile river valleys, leaving them unable to grow crops.

Glaucoma is caused when pressure builds up inside the eye, damaging the optic nerve that connects the eye to the brain.

It’s thought that 4.5 million people across the globe are blind because of glaucoma, making it the third highest cause of blindness worldwide. The condition can be difficult to diagnose because symptoms don’t appear straight away – instead, they develop slowly over many years.

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.