The Incredible Hulk is famous for being one of the first Marvel monsters who originally had a secret identity. Although the general public would eventually learn that the Hulk and Doctor Bruce Banner were one and the same, for several years, Banner managed to keep his regular transformations a secret from even his closest friends. Even after people learned the Hulk was actually a man who became a monster, few knew of Banner’s more disturbing secret; thanks to the gamma rays that mutated his body, he technically wasn’t even human anymore. This fact actually even applies to Banner’s human-looking form which, while apparently normal, is highly unstable.
Early Hulk comics had the Green Goliath snarl, “The Hulk isn’t human!” to his enemies. Most readers took this to be a sign of the Hulk’s prejudice against “puny humans,” as he frequently sought to distance himself from humanity in general and Banner specifically. Other versions of the Hulk had a different take on their personal humanity, with the intelligent Professor Hulk form affirming that he was human despite his green skin and musclebound form.
It wasn’t until the miniseries Ultimate Human by Warren Ellis and Cary Nord that the true damage to Banner’s DNA was laid bare. The Bruce Banner of this story was actually his Ultimate Universe counterpart, who became the Hulk through different means than his mainstream self; a failed version of the Super Soldier Serum that created Captain America. Eventually, this Banner came to Tony Stark and asked for help in finding a cure for the Hulk. Stark examined readings of Banner’s altered genetics and was flabbergasted by what he found. According to Stark, Banner’s DNA looked like “an Irish mother’s soup.” Banner admitted his DNA was constantly in flux and stated, “technically, I’m not actually human most of the time.” Stark even found synthetic hormones in Banner’s blood and commented that if Bruce ever tried to have kids, they’d have flippers and tentacles.
It’s a frightening admission, but one that makes sense (as far as comic book logic goes). In both the Ultimate and mainstream Marvel Universe, Bruce Banner’s DNA was irrevocably damaged by either gamma radiation, genetic engineering, or both. Yet while other superhumans like Spider-Man or Captain America who experienced similar accidents and experiments usually stabilized into new forms classified as “enhanced humans,” Banner’s mutated body is constantly in flux. Even when he looks like a “normal” man, his body doesn’t function like an ordinary human down to the genetic level. Things get even worse when he transforms, as he doesn’t just have a “secondary form” but a potentially infinite multitude of Hulk bodies, each with its own different DNA sequence - an idea Al Ewing's The Immortal Hulk series has been exploring to horrific effect.
Of course, some might argue that the definition of being “human” is more of a personal choice than a DNA sequence (over in DC Comics, Superman frequently makes that argument as he considers himself to be a human being despite his Kryptonian roots). Even in this respect, however, many of Banner’s Hulk personas don’t consider themselves human, and Banner himself admits the damage to his body has made him question whether or not he should be defined as human. Others may debate the issue, but in at least some respects, even the Hulk agrees he really isn’t human.