Assimilation

In the Star Trek fictional universe, assimilation is the process by which the Borg integrate beings and cultures into their collective. "You will be assimilated" is one of the few on-screen phrases employed by the Borg when communicating with other species. The Borg are portrayed as having encountered and assimilated thousands of species and (reportedly) trillions of lifeforms throughout the galaxy. The Borg identify species uniquely with a number assigned to them upon first contact.

Initially, the Borg were a mysterious group of marauders that snatched entire starships or took over entire planets and societies in order to collect and assimilate just their technology, being less interested in individual lifeforms. It is postulated that assimilation of individuals emerged as an efficient additional method for the Borg of gaining new knowledge and therefore enhancing the collective, as seen through the assimilation of Picard.

A Borg infant was found in the Borg cube, suggesting they reproduced rather than assimilated lifeforms. (TNG: "Q Who?"). This however is theorized to be their method of assimilating children and/or pregnant female sentients only. In their second appearance, "The Best of Both Worlds", they were shown to assimilate individuals—namely, Picard—into the collective by surgically altering them. This method is generally agreed to be the preferred method of assimilation in regards to fully matured biological lifeforms.
Jean-Luc Picard as Locutus after assimilation
Jean-Luc Picard as Locutus after assimilation

The method of assimilating individual lifeforms into the collective has been represented differently over time. Throughout, infant and fetal humanoids have been grown in an accelerated state and surgically receive implants tied directly into the brain, as well as ocular devices, tool-enhanced limbs, armour, and other prosthetics.

Later, in Star Trek: First Contact, the method of (adult) assimilation was depicted with the more efficient injection of nanoprobes into individuals. Borg nanoprobes are injected into the bloodstream of a victim by a number of tubules (usually two) that spring forth from the top of the hand (or some other extremity) of a Borg drone. The nanoprobes, each about the size of a human red blood cell (RBC), travel through the victim's bloodstream to various tissues and locations throughout the body and latch onto individual cells. The nanoprobes rewrite the cellular DNA, altering the victim's biochemistry, and eventually form larger, higher structures and networks within the body such as electrical pathways, processing and data storage nodes, and ultimately prosthetic devices that spring forth from the skin. Assimilation by nano-probe is depicted on-screen as being a fast acting process, with the victim's skin pigmentation turning grey with visible dark tracks forming where presumably blood vessels once existed.

In Mortal Coil, Seven of Nine states that the Borg assimilated the nanoprobe technology from "Species 149", suggesting that other improvements in assimiliation technology could have been assimilated, as well as indicating that the Borg's interest in assimilating technologies has not lapsed.

Assimilation is depicted as the preferred way for The Borg to gain information, especially about species of which no individuals have been previously assimilated. However, in several episodes drones are depicted as first interrogating alien technologies for tactical knowledge, rather than immediately assimilating the few individuals who may be present, as seen in Q Who.

Because assimilation depends on nanoprobes, species with an extremely advanced immune system such as Species 8472 are able to withstand assimilation.

The capability of nanoprobes to absorb improved technologies they encounter into the Borg collective is demonstrated in the Voyager episode Drone, where Seven of Nine's nanoprobes are fused with the Doctor's futuristic mobile emitter, creating a drone with enhanced capabilities to current drones, that then attempts to contact the collective. Fortunately for Voyager, this drone’s enhanced capabilities are not disseminated throughout the collective; the drone, in fact, sacrificed itself to save Voyager’s crew.

In William Shatner's novel The Return, Spock is nearly assimilated by the Borg, but is saved by the fact that he mind-melded with V'ger. This is because, according to Shatner's novel, the alien race that found V'ger was an earlier form of the Borg. Spock was saved from assimilation because he had part of the Borg Collective in his mind after he mind-melded with V'ger.