[5] As an adult, he became a geneticist working for Baron Helmut Zemo and Primus I, who later experiment on him and transform him into a cannibalistic, humanoid rat.

Under Venom's orders, Vermin later teams with Powderkeg and Mentallo; they temporarily defeat Iron Man and Hank Pym.

[5] Under continued treatment from Dr. Kafka, Whelan's original human form eventually becomes his default state, and he only changes to Vermin for a few hours at a time.

He and Kafka are kidnapped by a group of other Zemo mutants, who want Whelan to become Vermin again and lead them in their quest for revenge.

However, Zemo uses a mind-control device to take control of the mutants and revert Whelan to Vermin, intending to use them as subjects for further experiments.

Dr. Kafka convinces him to not kill Zemo, and by letting go of his hate Whelan allows "Vermin" to finally die.

No longer confined to psychiatric institutions, he submits to trial for his crimes as Vermin, with Matt Murdock agreeing to be his lawyer for the case.

[12] During the breakout of the Raft orchestrated by Electro, Vermin attacks Spider-Man after Count Nefaria knocks him into a hole with the other escaping villains.

Vermin, the Syndicate and an army of heroes attack and defeat an invading Skrull force in Central Park.

[21] During the Search for Tony Stark arc, Vermin rejoined Hood's gang and assisted in the attack on Castle Doom.

[22] During the Hunted storyline, Vermin was seen attacking people until he is defeated by Lizard and Taskmaster who hand him over to Arcade.

His agility, reflexes, and durability are roughly equivalent to the absolute peak attainable for the human body but, unlike his strength, speed and stamina, they do not reach superhuman levels.

Vermin has inch-long claw-like fingernails, razor-sharp pointed teeth, and fur covering most of his body, giving him the appearance of a humanoid rat.

Vermin has the ability to control rats and stray dogs within a two-mile (3 km) radius of his person.

Though Vermin has no formal martial arts training, he utilizes bestial ferocity, animal cunning, and instincts which make him a very formidable hand-to-hand combatant.

Following this, he took to living in a sewer system, where he developed a psychic connection to Agent Crock and become tyrants until they encountered the Young Ultimates.