Dr. Palance had worked as the X-Men's go-to physician for years. After discovering a serum that enabled him to use the powers of the multitude of mutants that he had treated over the course of his career, Palance became obsessed and began collecting a large sum of mutant samples. He eventually kidnapped Lady Mastermind and Omega Sentinel Karima Shapandar for unknown experiments. This attracted the attention of the X-Men, who had rescued the two women. Lady Mastermind and Shapandar then joined Rogue's team of X-Men with the promise that they would be able to take down the man responsible for their capture.
During the team's encounter with Palance (now operating under the name Pandemic), Rogue found out that she was Pandemics primary interest due to her memory absorption ability. He greatly wanted immortality and was convinced that through the means of her powers, he would be able to achieve some form of immortality by absorbing the memories of other people and thereby adding their lifetime's experience to his. Until then, he only had Rogues power of gaining powers.
To do so, Pandemic tried to extract the so-called Strain 88 (obviously the genstring with Rogue's mutant powers) and then injecting himself with it, so he could absorb her abilities. Before he could make of any use to her, Rogue's team of X-Men came to her rescue and defeated him. Lady Mastermind finally sought revenge against Pandemic and to give him a taste of how she felt put his mind in an endless maze, she left him like a rat scurrying for cheese in a science experiment.
Pandemic was also one of the nine villains that Beast contacted for help in his attempts at reversing the effects of M-Day, namely the decimation of the mutant population.
Powers and abilities
Pandemic created a virus that granted him the ability to copy the powers of nearly 300 mutants. His ultimate goal was immortality. Among other abilities he possesses power absorption (from Rogue), telepathy and telekinesis ( Jean Grey), teleportation ( Nightcrawler), elemental transmutation ( Alchemy), flame and heat generation and control ( Sunfire), omnimorphing ( Husk), ice generation and control ( Iceman), thermo-chemical energy generation and invulnerability ( Cannonball), rapid healing ( Sabretooth), combustive psionic energy from his torso ( Chamber), as well as at least 17 different forms of invulnerability.