Platyhelminthes, or flatworms, live in a variety of habitats, including in fresh and salt water, in soil, and in animal tissue as parasites. A majority of flatworm species are parasitic.
Parasitic flatworms that cause human diseases include the many species of tapeworms and flukes. These parasites enter the food chain through infected livestock, both through meat from infected animals and plants fertilized with infected manure.
Free-living species of flatworm do not cause disease, and some species are useful for pest control. Flatworms are used in Argentina to control malaria-carrying mosquitoes, and are used in Hawaii, Indonesia and the Philippines to control the spread of invasive snail species.