Interspatial parasite
Interspatial parasites are non-sentient microorganisms which derive from the transdimensional realm of the Sphere Builders. They exist in a state of temporal and spatial flux, and are capable of causing anterograde amnesia in humanoids.
Physiology
As a microorganism, the interspatial parasite is microscopic and is therefore too small to be seen by the naked humanoid eye. It is necessary to use magnifying technology to view them. They are irregular in shape but are generally composed of a thick, globular mass in the center, with between two and five 'arms' branching off from the edges. As well as being able to thrive within the Human brain, they can also be airborne and are capable of surviving transportation through spatial anomalies.
Interspatial parasites exist outside normal space-time, meaning that they are never fully in phase with the space they occupy but are instead in perpetual flux. They are still able to interact with the reality and time period they find themselves in but they are hard to detect and their nature makes it extremely difficult to affect them. There are only two known means of damaging or destroying them - shooting them with a focused antiproton beam powered by the warp core of a starship, or trapping them within a subspace implosion. However, since they never fully inhabit their environment and timeline, affecting an interspatial parasite in the present will affect it in the same way in all other places and time periods in which it exists. Therefore, destroying one means that it effectively never existed in the first place.
As well as being able to exist in air, interspatial parasites are also able to enter humanoid bodies and affect their brains. They do this by infesting the hippocampus and inhibiting synaptic functions, causing an extreme form of anterograde amnesia which allows the sufferer to remember events prior to the point of infection but which prevents them from converting new experiences into long term memories. Interspatial parasites which have infected a humanoid brain are able to remain there seemingly indefinitely, either by finding nutrients within the hippocampus or by reproducing and replacing themselves. Their effect on the host's synaptic functions never lessens and only stops if the interspatial parasites are removed or destroyed.
History
Interspatial parasites originate from another plane of existence, the transdimensional realm of the Sphere Builders, a species who created the Delphic Expanse and are bent on redefining the galaxy to suit their needs. Given the advanced technology of the Sphere Builders and their willingness to manipulate the development of lesser species, it seems possible that they were either responsible for creating the interspatial parasites, or that they shaped their evolution.
A collection of interspatial parasites travelled into this dimension in 2153 through a spatial anomaly within the Delphic Expanse which had been encountered by the Federation starship Enterprise, NX-01. The anomaly granted them access to the bridge of the ship, where they infested the brain of the Enterprise's Captain, Jonathan Archer. Since the presence of the interspatial parasites rendered Archer an amnesiac unable to forge new memories, he was removed from his position as Commanding Officer and replaced. However, without Archer to lead them, the crew of the Enterprise were unable to locate the Xindi superweapon they had been searching for before it was used to destroy Earth, killing the vast majority of Human life.
The Enterprise's Denobulan doctor, Phlox, spent years trying to find a way to remove or destroy the interspatial parasites but his efforts were continually fruitless because the parasites existed outside normal space-time. After consulting with the greatest Denobulan neurosurgeons and quantum physicists, he learned that they could be destroyed by a subspace implosion but this would also kill Jonathan Archer. After more than a decade, he found an alternative to this idea, the exposure of the interspatial parasites to a focused antiproton beam powered by the warp core of a starship. Boarding the Enterprise once more to achieve this, they were able to eliminate some of the interspatial parasites from Archer's brain, and discovered that since they are out of phase with space-time, destroying one in the present resulted in erasing them from the past.
They were unable to complete the process before the Enterprise was attacked by a Xindi fleet, so they decided to initiate a subspace implosion in the hope that it would destroy the interspatial parasites completely in all timelines, thereby creating a new one. They were correct and a new timeline was created in which Archer suffered nothing more serious than a concussion after encountering the spatial anomaly and was able to go on to save Earth from the Xindi.
It is unknown if this incident caused the extinction of the interspatial parasites as a whole or if there were more within the dimension of the Sphere Builders but there is no evidence that any continue to exist in this universe.
