People are undernourished because they cannot get food. People are restricted to their homes by ill health, neighborhood conditions, or other causes. People do not have enough money to buy the food they want. People do not spend available food money to get the best nutrition. Available food is not nutritious or is unappealing.
Nutritious food is too expensive so people eat substitutes which are less nutritious.
Lack of proper food causes illness. Deficiency diseases can result from malnutrition. Too much of some foods is related to certain health problems. Some diseases change the body's requirements for nutrients.