They can live inside the gastrointestinal tract or, the case of roundworms, in the lymphatic system.
It is because of your bodies little-known secondary circulation system called the lymphatic system.
1640s, from Latin lymphaticus, from lympha (see lymph). Also sometimes used in the classical Latin sense "mad, frenzied," which was the primary sense of lymphaticus in Latin: OED reports this "difficult to account for," but perhaps due to association of lympha with nymphe; cf. Greek nymphian "to be frenzy-stricken."
lymphatic lym·phat·ic (lĭm-fāt'ĭk)
Of or relating to lymph, a lymph node, or a lymphatic vessel. n.
A lymphatic vessel.