After separating from the Venom symbiote, Eddie Brock was not doing well. Believing he was suffering from cancer, he found himself in a downward spiral of religious paranoia, confusion, self-doubt, and fear of imminent death. After his last act of murder and attempted suicide failed due to the intervention of his old foe Spider-Man, Eddie had a change of heart. Although still dying and plagued by hallucinations of the symbiote, Eddie aimed to turn his life around and make the best of his remaining days. But fate had other plans for Eddie Brock.
In Amazing Spider-Man #568-569 (2008) by writer Dan Slott and artist John Romita Jr., Eddie Brock finds work with the FEAST (Food, Emergency Aid, Shelter and Training) shelter alongside Peter's Aunt May, under the employ of philanthropist Martin Li, also known as the super-powered crime lord Mr. Negative. Li's powers unexpectedly cure Eddie of his cancer and his Venom-ous hallucinations and, believing his clean state of health is a miracle, Eddie's new lease on life takes a turn for the worse when his past returns.
One day, the current Venom Mac Gargan shows up at the FEAST shelter looking for Spider-Man. Recognizing Brock, Gargan engages and taunts the powerless Eddie about rejecting Venom and its power, but his bullying is turned into shock and horror for them both as the symbiote attempts to re-bond with its former host. When the Venom symbiote attempts to return to Eddie Brock, an unexpected transformation occurs, preventing it from taking place. Unbeknownst to anybody, Martin Li's powers had caused the dormant remains of the symbiote in Eddie's blood to fuse with his white blood cells and Venom's reappearance triggered the creation of an all-new, color-inverted symbiote to emerge over his body. Dubbing himself Anti-Venom, Eddie worked alongside Spider-Man to fight Venom and the other members of the Thunderbolts who were acting under Director Norman Osborn's orders.
Throughout the remaining issues of the 'New Ways to Die' story-line, it is revealed that Eddie's new symbiote granted him many of the same abilities he had as Venom, along with a slew of new powers. Most notably, Eddie gained a tremendous healing factor with the ability to cure any sort of disease or impurity within a person's body due to the prominent antibodies within his system. Eddie's new skill set proved to be a blessing and curse depending on the situation and those involved. During his fight with Venom, his new powers proved almost fatal to the Venom symbiote and Mac Gargan spends most of the story arc in a modified Scorpion suit while the organism attemptsd to heal.
Anti-Venom also attempts to remove the symbiote's remains in Spider-Man's blood, but had to stop as the process would have also removed Peter's spider powers, continuing to dampen them during their temporary alliance. Armed with a new symbiote and powers, his alliance with Spider-Man deemed him a fugitive... but a newly repurposed Eddie Brock vowed to resume his vigilante anti-hero status in the pursuit of using Anti-Venom to help people. Anti-Venom would return sporadically during Dan Slott's run of Amazing Spider-Man until Eddie would make the ultimate sacrifice in 2011's Spider-Island.
After displaying that his Anti-Venom symbiote could cure the inhabitants of Manhattan that were slowly mutating into spider-monsters, Eddie chose to sacrifice the symbiote in order to synthesize a cure for those infected. Although Eddie would not be done with the symbiotes or Venom in the future, Anti-Venom would remain in that creative toybox of Marvel characters waiting to reappear in future Marvel comics.