Malnutrition is a general term to describe deficiencies in nutrition, generally of several nutrients at one time. It is common among the poor and is a major health problem in infants and children, even in developed countries.
Lack of food calories is usually the major component of malnutrition, but this alone will usually not result in major health problems. Often, people without enough food will also not be receiving one or more micronutrients (vitamins and minerals). Malnutrition is not unknown in people of high body mass, and is the result of a diet that is often too high in fats and carbohydrates and too low in vegetables.
Most of the resulting diseases from malnutrition are merely those related to specific vitamin deficiencies. However, prolonged malnutrition can have permanent effects and is particularly dangerous in the young. Moreover, a very malnourished individual may not react immediately to a better diet the way a person who is suffering from a single vitamin deficiency will. For example, an individual with scurvy will recover almost immediately when given vitamin C. A severely malnourished person may recover slowly, if at all.