Intestinal nematodes are roundworms; they range in length from 1 mm to many centimeters when mature (Table 217-1). Their life cycles are complex and highly varied; some species, including Strongyloides stercoralis and Enterobius vermicularis, can be transmitted directly from person to person, while others, such as Ascaris lumbricoides, Necator americanus, and Ancylostoma duodenale, require a soil phase for development. Because most helminth parasites do not self-replicate, the acquisition of a heavy burden of adult worms requires repeated exposure to the parasite in its infectious stage, whether larval or egg. Hence, clinical disease, as opposed to asymptomatic infection, generally develops only with prolonged residence in an endemic area and is typically related to infection intensity. In persons with marginal nutrition, intestinal helminth infections may impair growth and development. Eosinophilia and elevated serum IgE levels are features of many helminth infections and, when unexplained, should always prompt a search for intestinal helminths. Significant protective immunity to intestinal nematodes appears not to develop in humans, although mechanisms of parasite immune evasion and host immune responses to these infections have not been elucidated in detail.