Causal Agent:
Ascaris lumbricoides is the largest nematode (roundworm) parasitizing the human intestine.  (Adult females: 20 to 35 cm; adult male: 15 to 30 cm.)

Life Cycle:

Adult worms live in the lumen of the small intestine.  A female may produce approximately 200,000 eggs per day, which are passed with the feces .  Unfertilized eggs may be ingested but are not infective.  Fertile eggs embryonate and become infective after 18 days to several weeks , depending on the environmental conditions (optimum: moist, warm, shaded soil).  After infective eggs are swallowed , the larvae hatch , invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs .  The larvae mature further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed .  Upon reaching the small intestine, they develop into adult worms .  Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female.  Adult worms can live 1 to 2 years.

Geographic Distribution:
The most common human helminthic infection.  Worldwide distribution.  Highest prevalence in tropical and subtropical regions, and areas with inadequate sanitation.  Occurs in rural areas of the southeastern United States.