Parasites from a domain outside normal space-time that infest Jonathan Archer's brain in the Enterprise episode Twilight, leaving him unable to form new long-term memories.
Interspatial parasites exist partly outside normal space and time, which makes them almost impossible to remove by conventional medicine. In a humanoid host they settle in the brain and block the laying down of new long-term memories.
They appear in the Enterprise episode Twilight. In 2153 Archer is struck by a distortion from a spatial anomaly in the Delphic Expanse while pushing T'Pol clear, and is infected. Relieved of command, he lives for years with T'Pol as his carer while the Xindi destroy Earth and hunt the last human colonies.
Phlox eventually finds that a subspace implosion kills the parasites, and that because of their nature each one destroyed also vanishes from the past. Enterprise is sacrificed to wipe out the whole infestation, undoing the infection and the dark future it caused.