I welcome the privilege of writing some words of introduction to this important book. Its authors have been courageous in bringing together in one text a triad of topics that cover such large tracts of biomedical sciences as epidemiology, biochemistry, immunology, and clinical medicine. Malnutrition and infection are known to be closely linked, the one promoting the other. The adaptive immune system forms a part of the link since it is responsible for a good deal of defense against infection, and it may be affected adversely by malnutrition and indeed by infection itself. Knowledge in this complex field is of great potential importance because malnutri­ tion and infection are such dominant features of the ill-health of many of the world's underprivileged people. As this book shows, there is no lack of technical facets for study. There are now so many components of the immune response which can be measured or assessed and so many aspects of nutritional biochemistry which can be studied that the problem is to select what to study and where to begin. Moreover, the great number of variables in the nature of nutritional deficiencies, in types of infections or multiple infections and in the genetic, environmental, and social background of the affected people, all combine to make interpretation and application of findings a speculative business. Descriptions of cause and effect must us­ ually be provisional rather than definitive.