Sheep in Wolf's Clothing

A guileful hero can often avoid unnecessary fighting by disguising himself as a Trojan Prisoner, walking into an enemy base and planting explosives, rescuing the princess and laughing on the way home as the base blows up.

However, some enemies can't be fooled by first rate acting and good costuming. Some, like zombies, virus infected or robots, can instantly tell others of their kind on sight, smell, sound, taste, touch, Aura, or WiFi. Anyone attempting to traipse by through a throng of them will quickly be found out and eaten. There is one catch to this "foolproof", friend-or-foe recognition though. Basically, it assumes everyone who is one of their kind has the same agenda and interests, and when you go about infecting, robotizing, or metamorphosizing normal humans to your side, at least one of them is going to slip through the cracks, avoid or reject the Hive Queen's influence, and refuse to sell out humanity and become a Pro-Human Transhuman.

These Sheep In Wolf's Clothing use their early stage zombification/monsterhood to slip by enemy hordes and help their still-human allies however they can. This is usually a Heroic Sacrifice for a number a reasons: firstly, these Doomed Protagonists know the change is irreversible, and he or she will either die outright from the infection or suffer Loss of Identity once it finishes. Some heroes may infect themselves voluntarily to avoid detection, especially if the enemy is so well defended that only a suicide mission can end their threat.

If he succeeds, the hero can have one of three ultimate fates: become a monster and die (hopefully after succeeding in his mission), become a monster and live (which usually leads to What Have I Become?, but might not be so bad) or become human again thanks to a miracle antidote or No Ontological Inertia on the transformation.

Surprisingly not connected to A Wolf in Sheep's Clothing.

Examples of Sheep in Wolf's Clothing include:

Comic Books

In 30 Days of Night, Eben realizes that one of the few things that can kill vampires are other vampires. So he pumps himself up with vampire blood and utilizes his brief period of sanity to take on their leader.

Film

Ripley in Alien 3 survived an attack thanks to being infected, and later uses this to her advantage when confronting it later.
- Similarly, in Alien 4, she's able to kill Xenomorphs easily due to being a Half-Human Hybrid. There is also an infectee who puts his alien embryo to a good use by hugging a villain and letting the embryo tear through both of them.
Black Sheep has mutant carnivorous sheep, and IF you survive an attack, their bite will slowly muate you into a weresheep. The lead manages to cross a flock of sheep thanks to his infection.
District 9 has Wickus gain access to all Prawn technology 'thanks' to his mutating into one of them. This comes in handy when Cristopher's son sets a prawn battlemech to Kill All Humans mode.
Terminator Salvation has an interesting case. Though he is always a cyborg, Marcus was attacked by all Skynet robots when he believed he was "human." But after being outed in the resistance base he manages to infiltrate Skynet central in San Francisco because, thanks to this realization, the machines registered him as "Active".

Literature

Frank Herbert's Hellstrom's Hive. A secret agent is fed hormones that cause him to smell like the mutated members of the title organization, thus causing them to accept him as one of them. He infiltrates the Hive and causes a great deal of damage, until the hormones start to wear off and the members start to smell that he's an outsider.
In the Star Trek novel Resistance, Picard attempts to infiltrate a Borg ship by partially reassimilating into the Collective as Locutus. It doesn't work.
In Codex Alera, Tavi and Kitai can fool the Vord as long as the queen doesn't notice them and they don't attack any of the Vord. This is because the Vord Queen's form is based on their blood.

Live Action TV

In episode 5 of Supernatural's sixth season, Dean gets turned by a vampire seeking "pretty boy" recruiters for it's army. He manages to use this to his advantage in infiltrating the nest. This is less Heroic Sacrifice and more What the Hell, Hero?, as afterward Dean realizes that his brother Sam let him get turned just for this purpose.
- Mind you, he had an antidote back at base that he was like eighty percent sure would work so long as Dean didn't bite anyone. Not that he told Dean. Sam is kind of a dick without his soul.
The Voyager crewmembers try to infiltrate a Borg cube by trying a standard attack and getting assimilated, which turned out to be what they originally wanted. In the series finale, Admiral Janeway intentionally lets herself be assimilated in order to spread a Borg-destroying pathogen throughout the Hub.
In Angel the titular character used his condition as a vampire to infiltrate the Scourge. The Scourge at first despised him due to his human origin, but he convinced them that he loathed that aspect of himself and wished nothing more than to see humanity wiped out.
- Also comes up in the final season, in Angel's plan to infiltrate the inner circle of evil's operations on Earth.

Video Games

In World of Warcraft, the isolationist nation of Gilneas is turned into Worgen (werewolves) by a mysterious curse. Later on, they're attacked by the undead Forsaken (eventually the greater Horde in general), causing them to join the Alliance. Eventually, the Forsaken gain the ability to raise their own undead (previously they were only able to gather undead that were already raised by the Lich King), including the soldiers of the Alliance sent to fight them. The Worgen, being immune to the Plague of Undeath due to their curse, repaid the Alliance's efforts to save their people by taking the fight to the Forsaken.

Western Animation

Sonic's Uncle Chuck used his Unwilling Roboticisation to act as an inside agent for the Freedom Fighters. At least, he did when he was still in control of his mental faculties...