A mutagenic virus is a virus that mutates DNA sequences.
A Klingon male after the effects of the mutagenic virus
Centuries ago, 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")
See also: Klingon augment virus