The Xindi neural parasites were organisms used by the Xindi-Reptilians to ensure cooperativeness in their captives. They invaded the neocortex and reconfigured the synaptic pathways of their victim to ensure compliance. If untreated, the parasites lead to permanent neural damage.
Commander Dolim had Hoshi Sato injected with parasites to force her to decode the third launch code for the Xindi weapon. When Sato was recovered by Enterprise forces, the parasites proved no match for Doctor Phlox's osmotic eel. (ENT: "Countdown")