the Reploids have been retconned to only ever go maverick if they've managed to catch Robo-AIDS.
Yeah, too many retcons.
Original game, between the manual and Light's warning in the opening scene, suggests that what makes the Reploids turn on their masters is simply free will, and that the century X spent in the capsule was basically an incredibly thorough psych evaluation determining that, while capable of choosing between right and wrong, he'd choose right.
And we know Sigma's the head of the Maverick Hunters and switches sides.
X2 and 3 drop hints that Zero was created by Wily, and 4 finally spells it out overtly and, further, establishes that Zero was a Maverick, Sigma hunted him down, and after the fight they reversed roles.
So in Sigma's case, yeah, he's an inherently "good" guy who goes bad after exposure to the virus. Whereas Zero's designed as an antagonist but, after that knock on the head, he bucks his programming.
I don't think that NECESSARILY means all Mavericks have the virus (though I would not be the least bit surprised if that were explicitly stated somewhere in the PS run of the games). Some may have the virus; others may just be jerks. (And it's best to ignore the fact that 100% of all anthropomorphic animal Reploids are evil.)
From what I understand from reading Parish, Maverick Hunter X attempted another series of retcons, along with a book Capcom put out, and the whole Zero-as-virus-vector thing was being dropped. Which is a pity, given that it was pretty clever, but the X series got dropped at that point anyway.
I DO quite like the juxtaposition of Zero and Sigma as characters going opposite directions on the same path, one able to resist his programming and the other unable to resist being reprogrammed, and neither one of the fuckers able to stay dead.
(Of course, X5 muddies the waters even further with its branching plot; the "Zero goes bad" path makes a whole lot more sense and is more fun to watch, but of course you lose the ability to play as Zero. Whereas if Zero stays good, he and X just have a fight for no good damn reason. This could actually be fixed with some minor dialogue edits; X discovers Zero is the original virus vector -- possibly based on the fact that all the little virus avatars look just like him -- and tries to take him down, Zero responds by questioning X's judgement amid his increasingly erratic mood swings.)
Megaman ZX or ZXA, same thing really, does estrablish in a secret ending how the reploids came to an agreement on limiting their lifespan and how humans would get more cybernetics in to them, plus there's a bunch of other things that try to tie up into Legends, although far far in to the future.
They also follow both Mega Man Zero AND Mega Man X7, which is logically impossible. Best not to think too much about it.
