Ascaris lumbricoides is an intestinal parasite of humans and is the most common human helminthic infection worldwide, estimated to affect more than a billion people. Larvae and adult worms live in the small intestine of humans, and worm eggs are then passed in feces. Infection occurs when infective eggs are ingested, either from putting hands or fingers with contaminated dirt in the mouth or by eating contaminated fruits or vegetables that have not been sufficiently washed, peeled, or cooked. Outdoor defecation or the use of human feces as fertilizer can contaminate the soil with eggs. Eggs can remain viable through extreme temperatures and for several months in sewage.

While currently uncommon in the United States, infection with A lumbricoides (ascariasis) is still prevalent in developing countries. Risk factors include warm and humid climates, poor sanitation, and poor hygiene. Ascaris lumbricoides is thought to cause a major burden of disease worldwide due to acquired nutritional deficiencies (although some studies question whether the degree of nutritional deficiency is a significant factor in the development of disease). However, it is difficult to isolate the burden of disease of A lumbricoides specifically, since many individuals are coinfected with other parasites such as hookworms and whipworms. Young children (namely, preschool and school-aged) tend to carry higher numbers of parasites compared with adults, leading to growth retardation, impaired memory, and worsened cognition.

Ascaris suum is another species of roundworm that primarily infects pigs; humans can become accidental hosts, with organisms developing to the larval tissue-migratory phase but rarely maturing fully. Thus for the purposes of this review, Ascaris will refer to A lumbricoides, which represents the majority of human illness, rather than A suum.