The user can undergo mutation to receive heightened abilities and mutated physiology that may cause the user to lose their consciousness. The user hay have received various powers depending on what kind of infected powers the user has gained or what kind of infected monster mutated/infected the user body part.
Freeza (Dragon Ball) is a mutant amongst his species, a trait he inherited from his father, which accounts to their abnormally high power level and cruelty.
Utsuro (Gintama) is a mutated human born under the effects of Earth's Altana, remaining immortal so long as he remains on the planet to draw from the source.
Kouka (Gintama) is a mutated Yato born under the effects of Kouan's Altana, remaining immortal so long as she remains on the planet to draw from the source.
Spider-Man (Marvel) has become the Man-Spider.
Jugo (Naruto) in the process of mutating himself via his enzymes.
Sasuke Uchiha (Naruto) mutating his own body with the Cursed Seal of Heaven.
Kimimaro (Naruto) mutating his own body with the Cursed Seal of Earth.
Orochimaru's (Naruto) Test subjects
White Zetsu (Naruto) are humans who are mutated in a degrading manner from the "God: Nativity of a World of Trees".
The G-virus (Resident Evil series) is a highly-advanced strain of the Progenitor Virus created by William Birkin. The virus possesses fierce-regenerative abilities that revitalize the body, but also mutates its host into a horrible monster.
William Birkin (Resident Evil 2) was the creator the G-virus. After being mortally wounded, he infected himself with the virus, becoming a mutated creature of incredible strength, but slowly began to lose his humanity afterwards...
...as William became increasingly injured during his fights with Leon S. Kennedy and Claire Redfield, the G-virus rapidly healed and evolved his body to that a clawed juggernaut...
...In his third form, the G-virus mutated William in a hulking, multi-armed brute with his humanity all but lost...
...At this point in his forth form, William lost any resemblance to that of a rational being, transforming into a ferocious bipedal monster...
...In his fifth and final form, William has sustained significant damage to the point where the G-virus was unable to fully heal him, leaving him to become nothing more than a mindless mass of flesh and teeth.
Albert Wesker (Resident Evil) after absorbing the Uroborous
Alexei Stukov (Starcraft), the only infested terran to fully retain his original personality.
Chaos (Sonic the Hedgehog) was once a normal Chao, but was mutated into his current form by prolonged exposure to Chaos Energy.
Ninja Turtles (Teenage Mutant Ninja Turtles) were normal turtles before they came into contact with mutagen and mutated into anthropomorphic turtles.