The use of the theory of the general systems for studying the parasitic diseases has led to a better comprehension of the host parasite relationships and of the varying differences found in morbidity and biology of the parasitic diseases. Since environment has been considered an ecosystem of the individual, it can act upon man either by a direct action of the parasite on the host or as a determinant of food deficiencies. Among the consequences of the direct action of the infectious agent on the nutritional status of the host we can observe generalized catabolic disturbances. Also, secondary malabsorption syndromes and haematological alterations like aneamias of several types can be developed in the host's body by different mechanisms. The effects of undernutrition on the biology of the parasites are still better understood and, in the present paper, the Schistosomiasis mansoni was taken as an example because of its medical and public health significance in the nosologic picture of Northeast Brazil. Mice were fed with semi-purified and "natural" diets (similar to those consumed by the population living in endemic areas of Northeast Brazil where important endemic foci of this parasite are found). Finally, an analysis of the immunologic response in the pathobiology of nutrition /infection interrelationships was made, with special reference to the negative role of protein and iron deficiencies on cellular immunity.