Vitamin A deficiency is a nutritional disease caused by a lack of Vitamin A in the diet - generally diets that are very poor in non-starchy vegetables. It is very rare in the developed world, but affects nearly 500,000 people in the developing world each year. Vitamin A is a vital component of the light detecting cells in the retina and the immune system. It is very common in pregnant women (who have higher dietary requirements) and young children.
The first symptom is night blindness, which progresses to gradual and total blindness. In addition, a lack of vitamin A makes it harder to fight off infections, leading to a higher mortality rate with common infections, particularly measles.
Vitamin A is common in leafy and root vegetables, and supplements are also widely available.