In Marvel Zombies, how come he is susceptible to the bites and the virus? Shouldn't his healing factor stop the virus, or at least keep it at bay.

plot related reason

convulted bad writing to justify plot.

" convulted bad writing to justify plot. "

I'm pretty sure they said why in the issue pertaining to him and Kitty in the past (in Return), but i can't remember off hand.
but characters that are alot more resilient got the virus, as it had a very potent nature and could evolve around
almost any adversity. the only exception seemed to be characters whose physical cells weren't interconnected,
like Sandman and Manthing for example. I think it was because the virus worked faster than his hf, as it did to 
characters like Hulks as well, but don't quote me for sure on that. It's been awhile since I read that volume.
 

@Crom-Cruach

" convulted bad writing to justify plot. "

Just bad pis i mean Juggernaut also got infected i mean just unbelievable .

How about Hulk...does anything even pierce the hulks skin?

" How about Hulk...does anything even pierce the hulks skin? "