All right, all right! The point I'm trying to make is that Sigma and Zero are the only ones who consistently come back from the dead. Which in retrospect would have been much easier to word that way.

Aren't pointless debates fun, Mega X?

...okay, MY Sigma regeneration theories...

X1: Cain built him, he got the Virus from breaking Zero's head crystal. No crap.
X2: After X1 credits (that effect is SO old now), Sigma is still alive and ready to fight again, no help.
X3: He retreated into Doppler's body and had Doppler build him his new bodies
X4: He probably took one of the many other Sigma bodies in Doppler's hangar for this one, probably with a few modifications.
X5: He was able to reach the area where X and Zero were found after the Cannon crashed, and found either Wily or some data - preserved form of him, like the form of Light that found X at the end of X5, and Wily teamed up with Sigma to destroy X, building Sigma bodies.
X6: Sigma retreated to his Virus "hive" in Eurasia, where he waas able to find remnants of the Zero Virus in the colony. By merging with these, he was able to get into Gate and his comrades, infect them, and use his own Virus to put more power into the "Nightmare," and the reaction his Virus had on Zero was so strong Zero's body changed. From there, it was the same as with Doppler.
X7: Again, you're in a crazed scientist's lab looking for a new body. He just grabbed a bumload of guns, some tough armor, maybe the head left over from is unfinished first form, and went on his way. Into Red Alert.
X8: Axl's copy data was all in Red Alert, so finding his own data was no trouble. Plus, his body wasn't completely desroyed - it just fell. With it, he was able to last out until his Jakob Project began, (probably started during X5, when he was fully operational) and then he was able to power the Virus so much, it made its own body. Fom then it's obvious.
X9(?): Lumine walks over Sigma the same way Sigma did to Zero when he got the virus, and the result was the same, and all of the new generation reploids each had a complete copy of Sigma's Virus in them. X has a LONG way to go.

Yes, this is getting pontless, thank you RisingDragon.