A Klingon male after the effects of the mutagenic virus
A mutagenic virus was a virus that mutated DNA sequences, literally changing the physical makeup of the infected person from the DNA up.
Centuries before 2153, a mutagenic virus was created by the Loque'eque when they faced the threat of extinction. The virus was engineered to mutate the DNA of any humanoid visitors to the Loque'eque homeworld. When infected with the virus, the alien would also experience an extremely strong desire to return to the Loque'eque's home city, Urquat. Vulcan K-cells neutralized the pathogen which initially caused the virus. (ENT: "Extinction")
In 2154, a mutagenic virus infected the Klingon Empire. When infected with this virus, a Klingon's cranial ridges would dissolve. (ENT: "Affliction", "Divergence")
The Taresians lured unsuspecting men to their world by infecting them with a mutagenic retrovirus that turned them into Taresians and planted memories of the Taresian homeworld in their minds. (VOY: "Favorite Son")
In 2375, the USS Equinox's EMH, posing as his USS Voyager counterpart, claimed that the Equinox crew had been infected with a mutagenic virus, and he had been authorized to inoculate them. This was a ruse to release them from captivity. (VOY: "Equinox")
Brannon Braga regarded the concept of a mutagenic virus which altered one species into another as "actually a cool idea." (Star Trek: Communicator issue 151, p. 28)