Malnutrition dilutes the body's natural immunity and lowers the resistance to disease. Many clinical and biological studies have been devoted to the problem of the relationships between infection and immune responses among malnourished children living in those regions where there is a high incidence of protein-calorie malnutrition. The extensive involvement of the cellular and humoral defence mechanisms explains both the frequency and gravity of certain infections among malnourished children (as, for example, in measles patients), and contributes to the understanding of why immune responses to immunization are often inadequate.
A talk with the mother of a typically malnourished three-year old in a community of the developing world, says UNICEF, would reveal that in its short life the child had suffered perhaps 6 to 16 bouts of diarrhoea, 7 or 8 infections of the upper respiratory tract, 2 or 3 attacks of bronchitis, as well as measles and conjunctivitis and maybe - depending on the exact location of the village - an attack of malaria or meningitis. The detailed health record of 45 such three-year-olds, studied in the villages of Central America, showed an average, for each child, of one illness every three weeks. It is this mutually reinforcing relationship between invisible malnutrition and infection which is responsible for the majority of the 40,000 deaths every day among the developing world's infants and children.
In probably half of all cases, child malnutrition is precipitated not by the lack of food itself but by infection - especially diarrhoeal infection - which depresses the appetite and causes food to pass too quickly through the gastro-intestinal tract to be efficiently absorbed. That is why a child can sometimes be malnourished when there is food in the household and adequately nourished brothers and sisters are playing outside. More usually it is the interaction between malnutrition and infection which does the damage. Studies in Guatemala, for example, have shown that even a moderately malnourished child is three times more likely to contract a diarrhoeal infection than a child who is well fed.