He is not even closely related to Marvel and no.

i thought he was a virus that only looked human 

So he's a mutate, then.  Not a mutant.

" Sorry if this is a little off topic, but I've never even heard of him before & suddenly he's getting lots of threads dedicated to him :p  Why the sudden spike in popularity? "

Actually Alex is part of the DC universe so i think the question is how powerful would he be if he absorbed other villains and if the Justice League would allow him to be a member.

Lol... Alex can take the whole United States Army and the infected hordes on his own..He's a own man kind of guy and he can take care of himself. I very much doubt, with his attitude, he'll allie himself with the X-Men. I know there's other heroes and villans that can consume too but not like Alex. Though they said 'game', it is said that Alex Mercer is one of the most strongest video game character of all time, and they have comics based on him, so I think that will apply there too and he's just barely scratching the surface and already he's getting views. Alex doesn't need no JL nor X-Men unless they are helping him in some kind of way or whatever, but really? He got NY on a lock.

First of all if Alex Mercer from a repetative playing Prototype try to show up to pass for a mutant. Emma Frost and Cyclops would say exactly this. 

He wouldnt join them. doesnt care enough and doesnt have the attitude for it.