Marvel Zombies first appeared in Ultimate Fantastic Four where Zombie Reed tricked Ultimate Reed into opening a portal to that Zombie world.  Of course it is hyped that the 616 Marvel meets Ultimate Universe at that time.

Those Zombies F4 sneaked into Ultimate World and chaos ensures until it is solved.

Later Marvel Zombies series is launched.

Ultimate Fantastic Four : Crossover (#21-23) by Mark Millar and Greg Land

Reed receives contact from an older version of himself from an alternate dimension. After crossing over, Reed discovers he's been duped and that the entire world is infested with cannibalistic, zombified versions of the regular Marvel heroes, looking for their next meal on another world.

Ultimate Fantastic Four : Frightful (#30-32) by Mark Millar and Greg Land
The escape of the Frightful Four, Johhny has one week to live, and only Doctor Doom can help him.

Ultimate Fantastic Four HC 3 / Volume 5: Crossover & Volume 6: Frightful