A person's diet, or the nutrition they consume has a direct relation to quality of life and disease. Having a nutritious diet helps maintain a healthy body. If a person lacks a nutritious diet, say eating fast food every day, their body will show signs of this by way of weight gain, lethargy, irregular bowels, skin issues, etc. Fast foods lack basic nutrients and is often high in sugar, calories, fat, and carbohydrates. Your body needs these, but not in the quantity and not of the quality produced by fast food.
A person will begin to struggle with illnesses and diseases because their body simply isn't getting what they need causing the quality of life to decline.