(The following is based on my own personal canon, aka fanon, and is not to be taken as true Fallout canon of any kind other than possible RP scenarios. Enjoy.)
The Forced Evolutionary Virus Phase III, or just FEV III is the third and final amalgamation of the Forced Evolutionary Virus. It is a highly advanced successor of the FEV II, possessing fierce-regenerative abilities capable of essentially granting infectees biological immortality.
It was, ironically enough, accidentally created by Enclave scientist Dr. Lesko at a secret Enclave laboratory located in Michigan codenamed Conceive while splicing together two weathered FEV II specimens with an older, pre-war FEV I specimen.
As with the original FEV I and FEV II viruses, the Enclave considers the FEV III's primary means of infection to be through injections. To date, only two individuals have been witnessed administered a primary infection: two abducted test subjects, a second generation super mutant and a recently FEV-infected near-human male. Results were drastically different for each test subject.
Unlike mutants ever created by any of the FEV variants, III can pass infection onto other living organisms through physical contact and/or injury if it infects a non-mutant as it quickly renders them into a periodically growing, and hulking biological weapon of mass destruction. The effects are horrifyingly unpredictable...
The secondary infection was drastically differing. The FEV III possesses a unique reproductive aspect and is somewhat intelligent. If a genome of the FEV III is used on an already mutated super mutant, it will alter it into a breeding tool because of the excess FEV found in the DNA, and it will impregnate the infectee with an embryo in it's makeup transmitted through injection via the central nervous system with its reproductive organ.
In another, classified case, a host (an 'acquired' feral ghoul) who has no compatible DNA match will reject the FEV III parasite in a matter of minutes. This immunity has baffled Enclave scientists, unable to figure out feral ghouls' apparent rejection of the FEV and they hypothesize the same would likely occur in all other 'ghoul' specimens.
As a cautionary measure to the rather disturbing effects observed from the near-human infectee, all Enclave personnel have quickly been inoculated to it's effects and the FEV III stored for further study due too it's self-aware unpredictability...