Parasitic nematodes are tiny, worm-like organisms that live inside or on the bodies of various hosts, including plants and animals. They belong to the phylum Nematoda and can cause diseases in their hosts, leading to significant agricultural and health issues. These nematodes often feed on the tissues or fluids of their hosts, which can weaken or kill them. There are many species of parasitic nematodes, such as Ascaris lumbricoides, which infects humans, and Meloidogyne species, known as root-knot nematodes that affect crops. Control methods include crop rotation, biological control, and chemical treatments to manage their populations and minimize damage.