The giant intestinal roundworm, Ascaris lumbricoides, is a very common parasite with a worldwide distribution. The adult worms are 15 to 35 cm (6 to 14 inches) long and reside in the lumen of the small intestine. Sometimes, however, they are passed in the feces and, if vomited into the oral cavity, may exit from the host’s mouth or nostrils; thus they have been known to medical observers for millennia. Female worms produce up to 200,000 fertilized eggs daily, which are passed in the feces. Eggs incubate in the soil for at least 2 to 3 weeks to produce an infective larval stage within them. The eggs are very resistant to chemicals, desiccation, and extreme temperatures, but they mature or “embryonate” most rapidly in warm, moist, shady conditions in clay soils. People become infected by eating embryonated eggs in food or water contaminated with feces; or, in the case of toddlers, infection occurs by direct ingestion of eggs with dirt. Poor rural sanitation and the use of human feces for fertilizer obviously favor transmission. Mature eggs hatch in the small intestine, and the larvae then undergo a remarkable series of migrations in the host. They penetrate the intestinal wall and are carried in blood or lymph vessels to the liver and heart, and then the lungs. Here they break out into the air sacs, develop, and molt for about 3 weeks, and then climb up the trachea to the throat, where they are subsequently swallowed to establish themselves as adults in the small intestine.