Loeffler's syndrome is a rare form of pulmonary eosinophilia, with symptoms that are usually transient, self-limited, and benign lasting less than a month (usually 6-12 days). The etiology of Loeffler's syndrome is thought to be primarily an allergic reaction to the pulmonary migration of helminth larvae, specifically roundworm (A. lumbricoides), hookworm, and strongyloidiasis.

Pulmonary parasitic infection occurs as one of the pathogenic manifestations of the parasite's life cycle in humans. The body responds by increasing pulmonary eosinophilia, which is characteristic of Loeffler's syndrome. The cause of Loeffler's syndrome is related to various helminthic diseases, with different routes of transmission and pathogenesis.