In the later years of the X-Files, we were introduced to a seemingly new race of aliens that we can call “alien replicants” or “human replacements”. Though there are some similar characteristics to the regular ‘Roswell aliens’ in terms of their gestational stages (both start life as an infectious virus), everything else about these aliens is totally different. It’s questionable that we can even call them aliens. Their presence seems to arise from alien-human DNA experimentation under the direction of the original aliens. They are thus a unique hybrid-species with a supposed inherent instinct to stop all barriers to colonization. In terms of the examples we encounter, the life-cycle goes something like this:
| Viral stage – Like their alien gray progenitors, these aliens start as a kind of virus which infects a human host. It seems that this infection only occurs through a harsh and invasive surgical procedure performed upon humans. The first set to undergo this infection were military personnel—human guinea pigs that military scientists thought were part of an experiment to create a race of genetically engineered “super-soldiers”. (And later field tested in battle.) Afterwards, the hosts were intended to be UFO abductees matching a certain genetic profile, a fact which has been tracked under the auspices of the census program for decades. |
 Gestation – After infection, the virus lies dormant while its host is kept in a kind of stasis that mimics death (including rigor mortis and putrefaction). Once life support is given to an infected individual, however, the virus activates and grows. It somehow develops into a second and identical replacement of the original human from the inside out. This involves a sloughing of the original skin in warm water (not unlike that of the gray aliens in their infancy). |
 Replacement – Once ‘birthed’, the identical replacement of the human host is a fully aware and malevolent alien perhaps linked psychically to its kin. Physiologically, it is already quite different from other aliens because it bleeds red blood. Moreover, its body is not entirely organic–rather containing some metallic structure in at least one vertebra (giving the aliens a tell-tale bulge at the nape of their neck.) More significantly, these human replacements are virtually unable to be killed—self-replicating from the tiniest remaining portion of their metallic parts. |
 Mortality – Events at the end of the X-Files series revealed that the unkillable aliens can be killed. A certain magnetite ore seems to react with their alien physiology in a way that not only disrupts their system but disintegrates their entire body. It is unclear how or why this effect takes place, but it is probably what played a role in the ultimate defeat of this group. |