Nematodes (roundworms) are the most common parasites infecting humans worldwide. Of almost half a million species of roundworms, approximately 60 are known to be pathogenic to humans. Among the most prevalent human infections are those due to the intestinal (lumen-dwelling) nematodes. Ascaris lumbricoides and Trichuris trichiura each infect over 1 billion people worldwide; hookworms (Ancylostoma duodenale and Necator americanus) infect almost the same number. Other important nematodes of humans include Strongyloides stercoralis and Enterobius vermicularis. Coinfection, in particular with A. lumbricoides and T. trichiura, is common.

Ascaris lumbricoides, hookworm, and T. trichiura, collectively referred to as geohelminths (or soil-transmitted helminths), share the requirement for eggs or larvae to mature in soil in order to be infective to humans. Due to this obligate soil stage of maturation, these parasites cannot be transmitted from person to person. In contrast, S. stercoralis is able to complete its entire life cycle within the human host, and like E. vermicularis, both person-to-person transmission and autoinfection can occur.

The majority of geohelminthic infections are asymptomatic and associated with low worm burdens, whereas the minority (15%–35%) of infected individuals harbor the majority of the worm burden and suffer from more intense symptoms. Geohelminthic infections are important contributors to growth retardation and cognitive delay in children, but conclusively proving the benefit of large-scale anthelminthic therapy in endemic areas is challenging for a number of reasons. Geohelminths are unaffected by host immune responses, leading to chronic infection if untreated, although the natural history of such infections (excluding hookworm) is usually one of decreasing worm burden over time; even with treatment, however, reinfection is common.