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; hookworm (Ancylostoma duodenale and Necator americanus) infects almost the same number. Other important nematodes of humans include Strongyloides stercoralis and Enterobius vermicularis. Co-infection, 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. The majority of infections caused by these species 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. The natural history of geohelminthic infections is usually one of decreasing worm burden over time. Due to the obligate soil stage of maturation, these parasites cannot be transmitted from person to person. In contrast, S. strongyloides 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.