Malnutrition in a broad sense should include both insufficient or deficient nutrition and excessive nutrition. Only the former will be discussed here. Malnutrition is often secondary to some medical and surgical causes, such as chronic diarrhea, short bowel syndrome and malabsorption diseases. Non-medical causes of malnutrition are poverty and food shortages. Lack of nutritional knowledge and parents' neglect of scientific feeding methods. In developed countries, malnourished patients can usually be treated by treating the primary disease, providing appropriate diets, educating parents and carefully following up. However, in many third world countries, malnutrition is the main cause of death among children. There are complex interactions between malnutrition, social habits, environment and acute and chronic infections, so treatment is very difficult and cannot be solved simply by providing appropriate food.