This lifetime of several years is a result of their ability to manipulate the immune response of their hosts by secreting immunomodulatory products. The lifetime of adult worms varies tremendously from one species to another but is generally in the range of 1 to 8 years (see following table). Ringworm (dermatophytosis) is actually caused by various fungi and not by a parasitic worm. Helminths of importance in the sanitation field are the human parasites, and are classified as Nemathelminthes (nematodes) and Platyhelminthes, depending on whether they possess a round or flattened body, respectively. There may be as many as 300,000 species of parasites affecting vertebrates, and as many as 300 affecting humans alone. The phylum Platyhelminthes includes two classes of worms of particular medical significance: the cestodes (tapeworms) and the trematodes (flukes and blood flukes), depending on whether or not they have segmented bodies. However, for practical considerations the term is currently used to describe four phyla with superficial similarities: Annelida (ringed or segmented worms), Platyhelminthes ( flatworms), Nematoda (roundworms), and Acanthocephala (thorny-headed worms). The term "helminth" contains a number of phyla, many of which are completely unrelated. There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the nematodes. The term "helminth" is an artificial term. Helminths are a group of organisms which share a similar form but are not necessarily related as part of evolution. Image showing life cycle inside and outside of the human body of one fairly typical and well described helminth: Ascaris lumbricoides For example, Ascaris is the name of a type of helminth, and ascariasis is the name of the infection caused by that helminth. There is a naming convention which applies to all helminths: the ending "-asis" (or in veterinary science: "-osis") is added at the end of the name of the worm to denote the infection with that particular worm. An infection by a helminth is known as helminthiasis, helminth infection, or intestinal worm infection. Many of the worms referred to as helminths are intestinal parasites. The eggs can therefore survive in the environment for many months or years. These eggs have a strong shell that protects them against a range of environmental conditions. All parasitic worms produce eggs during reproduction. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the host's immune response by secreting immunomodulatory products. Parasitic worms cannot reproduce entirely within their host's body they have a life cycle that includes some stages that need to take place outside of the host. This can cause weakness and disease in the host, and poses a global health and economic problem. They receive nourishment and protection while disrupting their hosts' ability to absorb nutrients. Parasitic worms live in and feed in living hosts. Some parasitic worms, including leeches and monogeneans, are ectoparasites – thus, they are not classified as helminths, which are endoparasites. Other parasitic worms such as schistosomes reside in blood vessels. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Parasitic worms, also known as helminths, are large macroparasites adults can generally be seen with the naked eye. Eggs of different species of parasitic worm.
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