This paper reviews the major steps in alimentation, digestion, and absorption, which must be intact as a basis for normal nutrition, and discusses evidence relating parasitic infection in humans to effects on intestinal physiology. Parasites, with their ability to induce systemic toxicity and fever, to release active and toxic substances into the intestinal lumen, to compete for certain nutrients, to cause both functional and structural changes in the intestinal mucosa, and to stimulate hypermotility, which lessens the time available for digestion and absorption, can influence the alimentary process a almost every one of its steps. However, parasitic infection is likely to exert its most important impact at the very first step of the alimentary process, by adversely affecting the intake of food through any of a variety of mechanisms.