Several species of the genus Paragonimus, the lung flukes, can parasitize human beings. The most important, Paragonimus westermani, is found in China, Japan, Korea, Southeast Asia, Papua New Guinea, and parts of India and Central Africa. It was first discovered in the lungs of tigers in European zoos in 1878. Other species occur in Asia, in Africa, and in Central America and parts of South America. Wild and domestic members of the cat and dog families and other carnivorous animals are also hosts, and in many places humans are accidental hosts for worms that normally reside in other mammals. Adult worms produce eggs in the lungs, which reach fresh water either in the sputum or by being coughed up, swallowed, and passed in the feces.

Motile larvae hatch, penetrate an appropriate type of snail, undergo two reproductive cycles, and emerge to seek the second intermediate host, a crab or crayfish. Here they penetrate between the joints of the crustacean’s exoskeleton, and encyst there to await ingestion by humans or other definitive host. They then burrow through the intestinal wall and the diaphragm and enter the lungs, where they may survive for many years. Slow, chronic lung damage may become very serious in heavy infestations. Migrating flukes sometimes wander widely lost and reach atypical (ectopic) sites like the brain, where they cause a variety of neurological symptoms and may prove fatal.