A type of pathogen that alters those who are infected, also known more simply as a "mutagen." The sterile Loque'que of the Delphic Expanse tried to continue their species by engineering such a virus, designed to rewrite the DNA of any alien victim infected into becoming Loque'que. Long after the race is dead the virus survives to afflict any passers-by who become infected. Tret's species had long ago given up on finding a cure and fight the virus by simple eradication, but Dr. Phlox discovers that Vulcan K cells are not only resistant but neutralize the pathogen, and provide clues to a synthesized antivirus. In the end, Archer has the last vial of the virus preserved rather than destroyed — kept in stasis as a memorial to the longdead civilization.