Final answer:
Parasites, often transmitted through seafood or undercooked meat, have complex life cycles that may require multiple host species to complete.
Step-by-step explanation:
Parasites are commonly associated with seafood such as shellfish, but they can also be transmitted through undercooked meat like pork, beef, or fish. These organisms live either on the surface or inside their host and can enter humans through breaks in the skin, or more commonly, through ingesting contaminated food or water. The reproductive cycles of parasites, like that of tapeworms or the protozoan Plasmodium falciparum that causes malaria, can be very complex, sometimes requiring more than one host species to complete their life cycle.