Borg nanoprobes are microscopic machines that the Borg inject into their victims to assimilate them, rewriting the body with cybernetic components from within.
Assimilation begins when a drone injects nanoprobes into a victim, usually through assimilation tubules. The nanoprobes spread through the bloodstream, take over cells and begin building Borg implants, turning an individual into a drone linked to the Collective.
Voyager learned a great deal about the technology through Seven of Nine, whose body still contained nanoprobes after her separation from the Collective. The Doctor adapted them for remarkable purposes. In 2373 he modified nanoprobes into a weapon against Species 8472, whose biology resisted ordinary assimilation, and in 2374 Seven's nanoprobes revived Neelix after he had been dead for many hours.
The nanoprobes also allowed former drones to regenerate injuries, and medical researchers in later eras continued to study their potential. Their dual nature as instruments of conquest and of healing makes them one of the most significant pieces of Borg technology.