Causal Agent:
Metagonimus yokogawai, a minute intestinal fluke (and the smallest human fluke).

Life Cycle:
 

Adults release fully embryonated eggs each with a fully-developed miracidium, and eggs are passed in the host’s feces .  After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia which penetrate the snail’s intestine .  Snails of the genus Semisulcospira are the most frequent intermediate host for Metagonimus yokogawai.  The miracidia undergo several developmental stages in the snail, i.e. sporocysts , rediae , and cercariae .  Many cercariae are produced from each redia.  The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable fresh/brackish water fish (second intermediate host) .  The definitive host becomes infected by ingesting undercooked or salted fish containing metacercariae .  After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine and mature into adults (measuring 1.0 mm to 2.5 mm by 0.4 mm to 0.75 mm) .  In addition to humans, fish-eating mammals (e.g., cats and dogs) and birds can also be infected by M. yokogawai .

Geographic Distribution:
Mostly the Far East, as well as Siberia, Manchuria, the Balkan states, Israel, and Spain.