A bio-reactor was a type of medical technology which could be used for manufacturing viruses.
The Xindi-Reptilian scientist Damron and his team brought a bio-reactor with them when the Sphere Builders transported them back in time from 2153 to Detroit, Michigan in the year 2004. The Reptilians used the reactor, which utilized a neutronic power source, to synthesize a bioweapon with which to destroy Humanity. They were stopped by the intervention of Jonathan Archer and T'Pol, who were transported back in time by temporal agent Daniels. Upon returning to the 22nd century, Archer and T'Pol took the bio-reactor with them, later presenting it to Xindi-Primate Degra as evidence that the Sphere Builders had been deceiving the Xindi. (ENT: "Carpenter Street", "The Forgotten")