What is undernutrition? Undernutrition is a type of malnutrition caused by inadequate food intake or the body's inability to make use of needed nutrients. When individuals are undernourished, they can no longer maintain natural bodily capacities, such as growth, resisting infections and recovering from disease, learning and physical work, and pregnancy and lactation in women. Poor feeding of infants and young children, especially the lack of optimal breastfeeding and responsive complementary feeding, along with such illnesses as diarrhoea, pneumonia, malaria and HIV/AIDS, often exacerbated by helminths, are major causes of undernutrition.