From the ancient Greek ἕλμινς, "parasitic worm," "worm".

Helminths are a large group of organisms that cause over 150 parasitic diseases. Helminths include tapeworms or cestodes, flukes or trematodes (both groups are called flatworms), and roundworms or nematodes. Helminths are common in fish. Infestation occurs when fish eat infected molluscs. In turn, daphnids, cyclops and crustaceans ingest the pathogen with contaminated faeces from the environment. Some members of the ichthyofauna are not susceptible to worm infestation. These include whitefish, pike perch and pike perch. Animals and humans are usually infected with helminths through food and water containing eggs or worm larvae, or by eating meat of intermediate hosts.