Time from infection to oviposition is 65 to 90 days. The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with the stool. In the external environment, the eggs embryonate, hatch and yield miracidia which enter the first intermediate host, a snail. Ingestion of the metacercariae closes the cycle. Infections may persist for 20 years in humans, and occasionally other sites than the lungs are involved.
Infection occurs also in many animal species.