In Advent Children, a plague called Geostigma has spread across Gaia.
But then, what infected the Lifestream?
Cloud witnesses his mother’s death in the ensuing fire and rushes to the reactor to confront Sephiroth where he finds both a gravely wounded Tifa and a barely conscious Zack. Cloud picks up Zack’s Buster Sword, catches Sephiroth off guard in the reactor core, and impales him. When Cloud returns to attend to Tifa, a wounded Sephiroth emerges from the reactor core, but when Cloud attacks again, Sephiroth stabs him with his Masamune. Cloud summons previously unknown strength and uses the katana as a lever to lift up and hurl Sephiroth into the Mako pit below before collapsing.
This is also why I think Cloud goes off to look for a cure. He feels guilty for what he has done to the world and attempts to fix it. Cloud’s “stigma” is knowing he’s not SOLDIER AND what his sins to the planet.
In the end, Cloud has the same long sleeve covering his arm in Dirge of Cerberus. In Dirge of Cerberus he’s 24, which can be nishi (二死 – two deaths).
Then, who else?
In Dirge of Cerberus -Final Fantasy VII- the Omega Reports state that a naturally occurring deposit of similar Lifestream (termed “terra corrupt“) was found in Lucrecia’s Cave. How such deposits are created is not stated, but it is mentioned as being stagnant, and implied in Hoshi wo Meguru Otome and Case of the Lifestream Black that certain emotions in a spirit prevent it from flowing properly. This Lifestream forms a being known as Chaos.
And Vincent regularly visited Lucrecia’s Cave. So then, maybe Cloud also leads to Vincent’s poisoning, that is unless Vincent is immune.