Infections with A.lumbricoides occur by the accidental ingestion of mature, embryonated eggs, typically in food or on fingers. The egg hatches in the intestine, penetrates the mucosa and passes via the portal blood vessels to the lungs. There the larva breaks out into the alveoli and is coughed up and swallowed. If large numbers of larvae are present this can cause verminous pneumonia. The larvae then pass though the stomach again and grow to become adult worms in the small intestine. A female A.lumbricoides can grow up to 40 cm long and weigh 9 g. An adult male worm is about half the weight of a female and up to 30 cm long. Large numbers of worms can accumulate in the intestine which can cause intestinal obstruction, or they can migrate into the bile duct or pancreatic duct and cause obstruction there.
The distribution of worms among hosts is typically high aggregated, so that the majority of people have light infections while a small proportion have moderate to heavy infections. The distribution of A.lumbicoides and many other intestinal helminths is empirically best described by the negative binomial distribution. The heaviest infections with A.lumbricoides are typically found among children of school-age (5 - 14 years).
See ascariasis for more information.