Pretty much correct, Saber. The Sigma Virus is required for Sigma to revive himself after defeat, but Sigma is not required for the Sigma Virus to run rampant.
A relevant snippit on this exists in X5, after defeating Dark Dizzy. Alia speaks of Sigma lacking the energy to revive after his first battle, due to spreading the Virus over the world. Of course, Sigma obviously had prepared for his into-stage defeat (apparently Zero is a bit sharper than Alia on that one, he knew that Sigma lost on purpose), although this may go a ways as to explaining why Final Sigma W was very nearly the end of the line.
Speaking of which, the "isolation on the moon" argument fails in X8 for two reasons, the first of which X5 and X6 established: Zero. Although Sigma cannot control him, Zero is still a viable Sigma Virus host. It was because Gate found Sigma's data in his "piece of Zero" that Sigma returned in X6. That loophole still stands in X8. And the second loophole is Vile. He is an old-model Reploid and thus, unlike all of Sigma's other henchmen, completely vulnerable to infection. And his fate in X8 was not confirmed.
So there are ways the Sigma Virus could escape the moon. However, we know from X5 that the simple fact that the Sigma Virus exists does not mean that it is viable for Sigma's revival, even if the Virus is present for the battle, as X5 Sigma uses it to attack. It is possible that Sigma cannot survive if a large amount of the Sigma Virus is already being utilized for something else. Thus my belief, that it is being utilized in Belial Sigma's body.
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