More than 1 billion of the world’s population, including 4 million Americans, are infected. Together they have been estimated to pass more than 25,000 tons of Ascaris eggs into the environment annually. Like trichuriasis, with which it is coextensive, ascariasis is a disease of warm climates and poor sanitation. It is maintained by small children who defecate indiscriminately in the immediate vicinity of the home and pick up infectious eggs on their hands during play. Geophagia may result in massive worm loads. The para-site may also be acquired through ingestion of egg-contaminated food by the host; in dry, windy climates, eggs may become airborne and be inhaled and swallowed. In tropi-cal areas, the entire population may be involved; most worms, however, appear to be aggregated in a minority of the population, suggesting that some individuals are predis-posed to heavy infections. Isolated infected family clusters are more common in temper-ature climates.
There is convincing evidence that ascariasis induces a protective immune response in the host. Moreover, the severity of pulmonary damage induced by the migration of larvae through the lung appears to be related in part to an immediate hypersensitivity reaction to larval antigens.