The roundworm Ascaris lumbricoides is the most prevalent intestinal helminthic parasite, infecting well over one billion people worldwide. Each adult female can produce up to 200,000 eggs a day. Passed in feces, these eggs are remarkably resistant to environmental stresses and are capable of remaining viable for up to six years. With exposure to warm, humid soil, fertilized eggs become embryonated and infectious. Most transmission occurs by ingestion of fertilized eggs on dirty hands, in fecally contaminated agricultural products or other foodstuffs, or through geophagia.
Infection is more common in areas with poor sanitation.
In regions with large concentrations of Ascaris eggs in the soil, eggs may be disseminated in the air, inhaled, and swallowed later with respiratory secretions. From swallowed eggs, larvae hatch in the intestine within one to two days and molt into second-stage larvae, which are carried hematogenously to the liver and lungs. Roughly one to two weeks after infection, larvae from the capillary bed penetrate alveoli and molt into third-stage larvae, which ascend the tracheobronchial tree, are swallowed, and return to the intestine. The fertilized adult females begin producing eggs two to three months after the initial infection. The clinical manifestations of ascariasis occur during early larval migration or with established intestinal infections.