Interspatial parasite
From Memory Alpha, the free Star Trek reference
The interspatial parasite is an organism from the Sphere Builders' transdimensional realm.
In an alternate timeline, a series of parasites infected Captain Jonathan Archer's hippocampus in October 2153. The parasites entered his brain through a spatial anomaly and inhibited his synaptic functions, causing anterograde amnesia.
As the parasites were from outside normal space-time and existed in a state of interspatial flux, none of Doctor Phlox's treatments had any effect. Archer was relieved of duty while Phlox worked on a cure. The consequence of this was that Earth was destroyed by the Xindi superweapon in 2154, and Phlox returned to Denobula to work on a cure while Archer went with the rest of Humanity to Ceti Alpha V.
Phlox consulted with his world's finest neurosurgeons and quantum physicists. They concluded the only way to destroy the parasites would be a subspace implosion, which would of course vaporize Archer as well. A decade was spent developing an alternative way of dealing with the parasites.
In 2165, Phlox traveled to Ceti Alpha V with a cure. He discovered he could eliminate the parasites with a highly-focused antiproton beam powered by a starship's warp core. In removing the parasites, however, Phlox and T'Pol made an astonishing discovery: since the parasites existed outside of time, destroying a parasite in the present destroyed it in the past as well. They hoped that if they removed all the parasites, another timeline would be created in which Archer was never infected, and thus he would be able to save Earth from the Xindi.
Their work was cut short when Enterprise was attacked by Xindi forces seeking to wipe out the last of Humanity. With no option remaining, Archer and T'Pol vaporized Enterprise in a subspace implosion, destroying the parasites in all times, thus creating the new timeline and ultimately saving Earth. (ENT: "Twilight")
