The results of controlled experiments have demonstrated unequivocally that host food intake is often altered during the course of infections with eukaryotic parasites. Generally, host food intake is reduced depending on either the infective dose given to the host or the number of established parasites present. The onset in the reduction of host food intake varies according to the species of parasite involved and may be related to a particular developmental stage or event during the course of the host-parasite relationship. Some of the many sensory, neural, and hormonal factors that are now considered to modulate food intake in healthy animals are known to be affected during certain host-parasite relationships, and it is tentatively suggested that these physiological perturbations may initiate the observed changes in host food intake. Not unexpectedly, human appetite for food and food intake have been reported to become depressed during parasitic infections.