The nitrium parasite was a metal parasite encountered in 2368 by the USS Enterprise-D near Tessen III. At the time, the parasite was living within an asteroid composed of densely-packed nitrium and chrondite, which provided an ample food source. When the Enterprise was forced to destroy the asteroid in order to prevent it impacting Tessen III, the parasite transferred to the Enterprise as a new host.
Because most of the starship's systems contained nitrium in varying quantities, the parasite had ample food sources. In the process of metabolizing the nitrium, the parasite seriously endangered the functions of critical ship's systems including inertial dampers, life support, and the dilithium chamber. When the presence of the parasite was finally detected, in part due to its luminescent photonic trail, the Enterprise made its way to the Pelloris Field, a dense asteroid belt with high concentrations of nitrium. Once there, a particle beam rich in nitrium was directed toward an asteroid, providing a trail for the parasite to follow off the ship to another food source. Damage from the infestation was shortly repaired. (TNG: "Cost of Living")