Trichinella spiralis is an intracellular parasite as both a larva and an adult. The first-stage larva lives in a modified portion of a skeletal muscle cell, the nurse cell, and can reside there for the life span of the host. Adult worms occupy a nonmembrane-bound portion of columnar epithelium, living there as intramulticellular parasites. The newborn larva is the only nonintracellular stage, living free in the circulation. Trichinella spiralis induces modifications in each of its intracellular niches. Parasite signals secreted into the milieu of the developing nurse cell results in the reprogramming of host genomic expression, reflected in loss of muscle-specific proteins, over-expression of collagen, and the development of a circulatory rete. Formation of the nurse cell is complex, presumably involving many steps; yet there is not a large series of related intermediate forms in nature. Trichinella pseudospiralis induces an incomplete nurse cell. Adult parasites cause the death of the infected epithelium. The precise nature of most of the signals from parasite to host and from host to parasite has not been determined. As a direct consequence of exposure to some of them, the host develops long-lasting immunity to reinfection. This may confer advantages both for the parasite, as well as the host, because strong immune responses should reduce intraspecific competition.