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Parasitic worms or helminths are a division of eukaroytic parasites which, unlike external parasites such as lice or fleas, live inside their host.^[1] They are worm-like organisms that live and feed off living hosts receiving nourishment and protection while disrupting their hosts' nutrient absorption, causing weakness and disease. Those which live inside the digestive tract are called intestinal parasites. They can live inside humans as well as other animals.

Helminthology is the study of parasitic worms and their effect on their hosts.

Parasitic worms are categorized into three groups; cestodes, nematodes and trematodes.

Hookworms attached to the intestinal mucosa

The scolex, or head, of Taenia solium

Two pinworms, captured on emergence from the anus

Diseases caused in humans by helminth infection include ascariasis, dracunculiasis, elephantiasis, hookworm, lymphatic filariasis, onchocerciasis, schistosomiasis, and trichuriasis.

Parasitic worms are sequential hermaphrodites and reproduce depending on the species of worm, either with the presence of a male and female worm, joining sperm and eggs, producing fertile eggs, such as hookworms, or by breaking off segments that contain both male and female sex organs which are able to produce fertile eggs without the presence of a male or female. (e.g. tapeworms)

All worm offspring are passed on through poorly cooked meat, especially pork, wild fish, and beef, contaminated water, feces, mosquitoes and, in general, areas of poor hygiene and food regulation standards such as parts of Africa, Central and South America and Asia.

Worm eggs or larvae or even adults enter the human body through the mouth, anus, nose or skin with most species attaching themselves to the intestinal tract. With the presence of digestive enzymes, worm egg shells are dissolved releasing a brand new worm; unlike its egg shell, the parasitic worm is protected from the body's powerful digestive enzymes by producing a protective keratin layer.

The immune response to worm infection in humans is a Th2 response in the majority of cases. This results in inflammation of the gut and results in cyst-like structures forming around the egg deposits throughout the body.

1. ^ Maizels RM, Yazdanbakhsh M (2003). "Immune regulation by helminth parasites: cellular and molecular mechanisms". Nat. Rev. Immunol. 3 (9): 733–44. doi:10.1038/nri1183. PMID 12949497.