Reaching the lungs, the larvae burst into the alveolar spaces, where they undergo the next molt to the L4 stage. After a period of development in the lungs, the L4 migrate up the trachea and are swallowed and pass down the oesophagus, through the stomach and into the small intestine. Here they undergo the final molt to produce the young adult stages. The adult hookworms attach to the mucosa of the small intestine with their powerful modified stoma, abrading the mucosa until they puncture the blood capillaries and can suck the host blood. Heavy hookworm infections can account for as much as 200 ml of blood loss per day.
Individuals become infected, usually by walking bare footed across contaminated soil.
Soil contamination can be extremely heavy, particularly in those cultures which utilise human excreta, known as Night soil, as fertilizer. The eggs are susceptible to urine however, and it has been found that if faecal material is mixed with urine the eggs can be killed. There has also been the suggestion that juveniles can survive in the muscles of an abnormal paratenic host and can resume normal development when the infected meat is eaten raw or poorly cooked. When larvae initially penetrate the skin they can cause irritation and itching and if they fail to locate a blood capillary they can wander through the skin causing a condition known as cutaneous larval migrans , leaving a track visualized under the skin by the presence of the host inflammatory reaction. This causes local haemorrhaging at the site.