Hulk is the result of gamma radiation exposure to Bruce Banner.
History
Scientist Bruce Banner’s life would change forever the day he tested his highly dangerous Gamma Reactor for Stark Enterprises. When he noticed a young teen named Rick Jones on the testing range, he ran onto the field in order to remove the boy. Initially thinking the reactor countdown had been delayed, he saved Rick but was too late to save himself and Banner’s body was bombarded with an immense amount of gamma rays.
The gamma radiation altered Bruce’s DNA, causing him to become The Hulk, an unstoppable brute with no apparent limit to his strength whenever he became angry or stressed. The creature’s mind exists separate from Banner’s, with Bruce having no control over it and remembering nothing of his time during the metamorphosis.
Realizing that the Hulk is a threat to everyone around him, Banner began desperately working on a cure for his condition, whilst General Thaddeus Ross believed Banner to be a threat, causing Banner to run and act as a fugitive whilst he worked on his cure.
The man who originally sabotaged Banner’s gamma reactor experiment, Samuel Sterns, ironically also became a victim of the radiation itself, whilst on the clean up crew; he fell into a puddle of gamma waste. This gave him increased intelligence and a thirst for power. Believing Hulk was his creation, Sterns, now calling himself Leader, kidnapped Banner, and planned to steal Hulk’s strength for himself, giving Banner yet another reason to look over his shoulder.
After Reed Richards of the Fantastic Four believed their own transformation was somehow connected to gamma radiation, he asked Dr. Banner to help him research the cause, in hopes of curing Ben of his Thing form. Doctor Doom kidnapped Bruce Banner and tricked Hulk into thinking the Fantastic Four had stolen Rick Jones’ friendship. They battled, but Hulk soon realized he was being duped, and attacked Doom. Bruce and Rick then left New York City.
When Banner’s nemesis, Leader, attempted to steal Mandarin’s rings in an attempt to travel back in time to gain the Hulk’s strength for himself, Iron Man and Hulk intervened and battled in several different periods of historical importance, whilst Iron Man searched for Julia Carpenter, who was trapped in the past.
After completely destroying Iron Man’s armor, Hulk realized that Leader was his true enemy, not Iron Man.
After several failed attempts to cure himself, Banner’s girlfriend,Betty Ross and comrade Doc Samson created a nutrient bath, hoping to separate Banner and Hulk completely. After the test was interrupted by Major Glenn Talbot, the operation left both Hulk and Banner as vegetables. Eventually waking up, Hulk, now a mindless beast, began destroying everything in site, whilst Banner was bombarded by guilt. After attempting to finally defeat Hulk, Banner got him angrier than ever before, and they both collapsed. Eventually Leader kidnapped Hulk and attempted yet again to transfer his power into his body but an incident occurred and Hulk absorbed Leader's intelligence and powers, his head grew big. When Hulk awoke he was possessed by Leader.
Leader possessed Hulk went on a rampage at Gamma Base where Banner was and wanted to capture him. But eventually the transfer went wrong and Hulk was back in his body whilst Leader was normal again. Leader was defeated by Hulk and She-Hulk. Tests showed that Hulk and Banner simply couldn’t survive without each other. Samson and Betty attempted to reverse the Nutrient Bath, but an intervention from the insane General Ross caused Rick to fall into the Bath, leaving him as a version of Hulk. The stress caused another startling transformation in Banner; he turned into a Grey Hulk, with a personality completely different to both Banner himself and the more savage green Hulk.
The insane Ross attempted to defeat the new Hulk, but their battle left Ross in a coma. Leader then promised Grey Hulk he would remove Banner permanently, if Hulk could capture Rick Jones. After capturing the confused teenager, Leader tricked Hulk and tried to steal his powers. The experiment was a failure and Rick-Hulk strength was transferred into Grey Hulk. This event left Banner with three different personalities. Banner himself, the savage Green Hulk, and the arrogant Grey Hulk.
Bruce was possessed by a Dark Entity who sought the world's most powerful being. Bruce then transformed into the Dark Hulk. She-Hulk and Doctor Strange had to go into Bruce's mind to stop them where they were assisted by the mental personalities of both the Green and Grey Hulks.
After hanging out with his cousin She-Hulk for a while, Banner reconciled his differences with General Ross after Thaddeus and Betty became possessed by a parasitic creature.
At some point he joined the Defenders and their adventures were heard about by Peter Parker. At some other point, Hulk joined the superhero team called the Avengers. However, he left and a portrait of him was put up in their conference center.
The X-Men, realizing they may have to fight Hulk at some point, designed a robot after Hulk for their Danger Room practices. When Juggernaut attacked the Xavier Mansion, Charles Xavier fled to the Danger Room and used the Hulk robot to fight his step-brother, who defeated it easily.