Threadworms in the gut. Coloured scanning electron micrograph (SEM) showing multiple threadworms (Enterobius sp., red) on the interior of a human intestine. Threadworms are nematode worms that parasitise the large intestine and caecum of many animals. In humans they cause the common infection enterobiasis. Nematodes are unsegmented worms that may be parasitic or free-living depending upon the species. Magnification: x800 when printed at 10 centimetres wide.