The route seems needlessly elaborate. A larva that hatches in the small intestine penetrates the intestinal wall, enters the portal circulation and reaches the liver, then travels via the heart to the pulmonary capillaries. There it breaks into the alveoli, moults, and ascends the bronchi and trachea to the pharynx, where it is swallowed and returns to the intestine to become an adult.
The lung phase has a clinical name. Larval passage through the lungs can cause a transient eosinophilic pneumonitis, described as Loffler syndrome, with cough and wheeze appearing days to weeks after ingestion and typically resolving on its own.
Why the migration exists at all is not settled. One long-standing suggestion is that it retraces an ancestral route from a skin-penetrating ancestor; another is that the larva requires a growth phase in oxygen-rich tissue. Neither is proven. What is clear is the scale of the parasite: ascariasis is estimated to infect hundreds of millions of people, with the heaviest worm burdens in children.