Pathogenesis - How does Ascaris Lumbricoides causes illness?

Ascariasis is infection caused by Ascaris lumbricoides, an intestinal roundworm. 

First, the eggs of the worm are found in soil contaminated by human stool or in uncooked food contaminated

by soil containing eggs of the worm. 

Second, person can get infected when he/she swallows contaminated food with Ascaris eggs. It usually occurs in poor sanitation area.  

Third, once swallowed, Ascaris eggs hatch and release larvae in the intestine. 

Then, each larva migrates through the wall of the small intestine and is carried through the lymphatic vessels and bloodstream to the lungs. 

Fifth, in the lungs, the larva passes into the air sacs. It then moves up to the respiratory tract and into the throat and is swallowed.

Finally, the larva matures in the small intestine, where it remains as an adult worm. This is the stage where variety of symptoms could occurs if the worm burden is heavy. They might be huge enough to form a bolus and obstruct the intestine lumen or they might migrate to other places of the body due to some stimulation.

Female adult Ascaris Lumbricoides produce eggs. Eggs laid by the adult worms are excreted in stool and cause infection again when they are ingested by another person.